a) The social & political implications of the geography, technology and magical abilities.
b) The likely differences between warfare in this setting and 19th century historical conflicts.
c) Any setting detail likely to influence the personal history / occupation / outlook of the main characters.
d) The plausible military and poltical responses to the extinction-level event in the main story.
e) Best rationalisations for the most suspension-of-disbelief threatening elements (e.g. the airships).
Generally if anyone has neat ideas for fantasy elements that fit with the setting, I will try to include them.
Geography
Windhaven has significantly more ocean cover than Earth; both poles have landmasses somewhat larger than Antarctica, but the rest of the land consists of endless chains of small islands. Climate is similar to earth but storms are somewhat more common, and the atmospheric dust loading is higher (insolation might be higher to maintain the same temperature?). There is a moon similar to earth's but somewhat smaller, so tides are about half as strong. Plant and animal life is very similar to contemporary Earth; my background notes have the planet terraformed about ten thousand years ago, though this is not known to the characters and never referenced in the story. The game takes place in a chunk of the northern hemisphere;

The story visits four locations;
Taahay - a western temperate island settled in the last 100 years with a 'frontier' feel to it.
Kyvitoya - a snowy, rugged island in the far north, close to winter extent of the ice pack; lots of jagged peaks and deep valleys
Atieu - volcanic island near the equator, with tropical climate and central lagoon
Fillicoudi - one of the most populated islands, split roughly equally into city, fields and marshy badlands.
Magic
Roughly one in three thousand humans on Windhaven have an innate ability to use magic. The trait is not directly inherited, but the incidence of the children of magic users having the ability is much higher, roughly one in one hundred. Recently this has been proven to be caused by the child being conceived on an island with the ancient enchanted monuments, as distant outlying islands with no such monuments have been colonised and no one born their has magical abilities. I am not sure what the best terminology (formal and slang) for these people is.

All such individuals have the ability to transform themselves into a specific kind of bird; the species seems somewhat related to personality. This is the first ability to manifest, in the early teens. Transformation takes a couple of seconds, is painless and has assorted lightshow visual effects. It initially doesn't include clothing/equipment, but with experience shifters can take along anything they can carry. If killed while transformed the person returns to human form about ten minutes later. Aside from natural abilities in bird form shifters have supernatural resilience and fast healing; being hit by an arrow or a musket ball will cause minor bruising, being ten feet from an exploding naval shell will cause severe lacerations and internal bleeding that heals after ten minutes or so, a direct cannon hit is still instantly fatal.
Each magic user also gets a set of abilities with a particular theme, that they can develop with practice and training. These magical abilities require only an act of will to use, although many practioners find words and gestures make it easier. The effects achievable are the same in human or bird form and all of these abilities are mentally fatiguing, which would restrict use to half an hour or less per day. However in bird form shifters can make use of shrines; these are a particular kind of monument found in large numbers around all the inhabited islands. Flying through these instantly heals the shifter, removes all fatigue and charges them with some kind of energy that dramatically reduces the 'casting time' to trigger an ability, from a minute or more to a few seconds. This energy lasts for a dozen or so uses of an ability, or until the shifter returns to human form.
Abilities seen in the storyline; probably near the high end of abilities, but it's implied that these are just some of the most useful expressions of the basic ability, not the only things the characters can do;
(video with all the player character abilities in it)
* All magic users seen in the story have the ability to communicate telepathically with each other, in human or bird form. This works like speech, no apparent way to communicate images, complex thoughts etc. Range seems to be about ten kilometers, not enough to reach between islands. In bird form all shifters can also sense flying objects, large moving objects on the ground, terrain topography and magical effects out to several kilometers. This is a 360-degree 'always on' ability, although shifters can miss details they aren't specifically concentrating on just as with vision. These basic abilities can be used concurrently with anything else, however active abilities can't be combined and it takes some time to switch between one effect and another.
* Creating a glowing ionised channel out to 200 metres, and then shooting 'ball lightning' down it, which will stun or kill a human on impact. This is a very common and quite easy to use ability (it's the starting attack for all characters in the game). Effectiveness on the ground is limited by the fact that the lightning will arc to earth, so practical range is only 10 to 50 metres depending on ground conductivity. Full range is always available for air-to-air and air-to-ground attacks. Sustained fire rate is approx every two seconds, which can be maintained for half an hour plus even without charging at shrines.
* Deajan can create magical crystal barriers that pass airflow and light but stop all physical attacks up to a naval shell. They ablate (shatter) on impact but then regrow as long as he concentrates. He can either mentally hold them in a position relative to his body or shoot them forward as projectiles. On foot they take minutes to form and grow, as a bird with a shrine charge they take seconds. Max surface area seems to be about 10 m2 for a constant shield, at least 100m2 for a fired projectile. Deajan can also create basketball sized lumps of red hot rock and shoot them at 100 metres/second or so. These contain pressurised volcanic gas that makes them explode on impact (or after travelling a few hundred metres). Without a charge it takes a minute or so to make one. With a shrine charge he can make twelve at once in a few seconds, telekinetically hold them until firing, and shoot about ten salvos before needing to recharge. In the cutscenes Deajan says that he uses his abilities (in unspecified ways) to help with mining and construction, formerly he used them for military purposes.
* Sakoti can shoot gouts of flame out to 50 metres sufficently hot and dense to set several trees completely aflame with a two second burst. Cutscenes imply that in human form this is limited to more like 10 metres and covering one or two people. She can create dense balls of flame that home in on a particular target, travel up to a kilometer (taking ten second to do so) and explode with the effect of a 500lb napalm bomb. The homing is not very effective and the shots can be dodged by other fliers, cavalry or anything similarly agile. With a shrine charge she can make about thirty of these fireballs taking two second to charge each one. Sakoti seems to have remote sensing abilities beyond the usual 'radar vision'; she is able to do a superficial analysis of the anatomy and biology of extradimensional creatures by concentrating on them for a few minutes.
* Alaysia can create dense pre-nucleated rainclouds that drench a roughly 200m x 200m area with 'holy rain' for about thirty seconds. This stuff burns extradimensional creatures like acid, improves crop growth and
seems to partly heal and regenerate burned-out trees; other than that it's like normal water. She can also create tornados, which only last for twenty seconds or so unless the right weather conditions are present
to sustain them naturally. These abilities take a minute or two to use in human form, seconds in swan form with a shrine charge, ten to twenty such uses per charge.
* Lyozar can create strong electrical charges on objects in the vicinity; the charges are randomly positive or negative, so the result is huge arcs of electricity leaping about and burning/stunning/vaporising the
targetted objects. This seems to affect everything in a cone out to fifty meters or so, with some control of range and spread. With a shrine charge this is 'instant on' and can be maintained for thirty seconds or
so. As with the basic lightning attack range is greatly reduced on the ground. He also has an 'ether flight' ability that increases his flight speed to high subsonic (at the cost of near-zero maneuverability), for up
to ten minutes or so.
* We meet a woman who can heal severe trauma and pretty much any illness by touching the patient and concentrating for a few minutes. When the island is under severe attack, she uses a shrine to charge up
and completely heals a string of badly burned people by hovering over each for a couple of seconds and glowing (as an albatross).
* Later a densely populated island is being protected by a flock of pacifist owl-shifter monks projecting 'fire shields'. These are glowing hemispheres about a kilometer across that burn monsters to ash on contact
and are able to withstand the equivalent of a light artillery barrage for about ten minutes before collapsing.
* Large numbers of vulture-shifters come after the protagonists. Most of these are criminals who have been given the ability recently by a bizarre alien process (that also gives them incurrable cancer). Most of the
ones who were previously shifters had their bird form change species to griffon vulture, which implies that sufficiently deep belief/personality changes or some kind of ritual can do this. Some of them have the ability
to weaken dimensional barriers enough to allow monsters to cross to Windhaven.
Teleportation, mind-reading/control and time travel abilities do not exist in this setting.
Magical weapons exist, such as impossibly sharp blades and cannon balls that create an incendiary explosion around themselves on impact. These are created by shifters with a talent focused on enchanting items, none of whom are currently seen in the story. They are too expensive for everyone to have one but it is practical to equip every soldier in an elite unit with scale armour that is magically proof against musket balls, so I assume they take days to months to make (depending on the item). Once enchanted the effect seems to last indefinitely, expect for one-shot bombs and ammunition. Magical steam engines also exist that require no fuel; due to the very limited supply, these are almost entirely reserved for powering airships. There is a general theme of magical items being impossibly strong and able to create and destroy heat and electrical energy, but not able to move on their own or exhibit any kind of intelligence. Other than enchanted items, magical effects never persist beyond when the caster stops concentrating on them.
History
The defining event of Windhaven's history is the near-total destruction of planetary civilisation nine centuries before the present. Humans on Windhaven had achieved a population over one billion and technological advancement comparable to mid 20th century Earth, but augmented sophisticated magical abilities used by both individuals and created devices (with a feel similar to the atomic super-science of 1950s sci-fi). Contemporary historians have three major theories for this; a deadly plauge, a global war with devestating weapons and an attack by demonic creatures. All of these have some supporting evidence and the truth is a combination of all three.
Cities were reduced to rubble, large concentrations of humans consumed, many more died from starvation and finally a deadly virus killed over 90% of the survivors. In the century following the cataclysm, global population dropped to barely a million and society regressed to something comparable to dark ages Europe. I haven't detailed the following half-millenium, but it presumably consists of warlords carving out kingdoms, ongoing intermittent warfare (island-hopping along chains), rise and fall of noble families, the rediscovery of isolated communities etc. In the past three hundred years there has been a rise of nation states, increase in population and surge of academic and technical development equivalent to the renaissance. The most advanced areas are well into an industrial revolution, although rail has not developed beyond a few mining installations due to the massive trade fleets, canal networks and lack of long land routes.
I have some very rough sketches of assorted nation states that exist in the world, but I'm refraining from posting them because I'd like to see if anyone has interesting ideas for late-18th/early-19th century fantasy nations. The storyline includes three particularly large nations in a tense three-way standoff, with an alliance about to be formed between two of them and the third paranoid and desperately searching for smaller allies to maintain a balance of power. The lead character played a major role in an attempted invasion of his own mid-sized kingdom four years earlier, so there are definitely plenty of land grabs and flare-ups going on. Presumably there is plenty of piracy, raiding and corsairs going on as well, although this doesn't feature in the story. The main villain in the story is a discraced politician, possibly survivor of a failed coup d'etat before becoming a cult leader; still looking for ideas for his backstory.
I would like to have the relationship of magic users/shifters change from taking positions of personal power themselves, to being more of their own social class, considered a valuable resource by national leaders and coexisting uneasily with the various forms of nobility and merchant classes around the world. Since they frequently come from peasant backgrounds and are uniquely gifted and mobile, they should be somewhat mercenary and insular, with their own factions and training academies. The relationship with the common people should be a tricky mix of respect, jealousy, fear and hero worship, considering that bird-shifters are the only means of rapid communication and the only source of really effective medical treatment. All the characters seen in the story are non-sexist by author fiat, but one can presume that since 50% of the rare and valuable magic users are female, an exception is made to historically typical sexism in this case.
Technology
We don't see a lot of this in the story, and what we do see has a rather wide range of sophistication, a possible plausibility issue. The navies of Windhaven are composed mostly of multi-masted cannon armed ships similar to late 18th century ships-of-the-line. The major nations have a dozen or so ironclads each; the ones we see look like enlarged (three times the displacement) versions of the Monitor and the Virginia. Deajan commanded a unit armed with rifled muskets; at one point Lyozar mentions stealing blueprints for a prototype 'self-loading volley gun'. In various interiors we see candles, globes, pickled specimens, in the richest areas gas lights. The nations of Windhaven seem to have made less progress on global mapping than Earth did with the same technology; possibly because the stronger storms make crossing the wider stretches of ocean more hazardous (for both ships and bird-shifters). From the ease with which they start shooting at flying monsters, it seems that Windhaven navies are used to having to open fire on enemy bird-shifters, although the prospects for bringing one down with grapeshot out of late 18th century naval artillery seem somewhat limited.

By far the most advanced things seen are the airships, which look like first world war Zepplins. They are powered by lightweight steam engines, filled by hydrogen, exist in relatively small numbers and are considered
valuable enough that major nations hesitate to risk them rescuing populations from monster attacks. They are able to carry hundreds of people and probably use lots of magically enhanced components beyond the engines. In the final mission we see a couple of prototype airships equipped with 'lightning shields' and 'death rays', which work pretty much the same as they do in Flash Gordon. This is presented as directly reverse engineered from the ancient rings/obelisks based on the work of Sakoti and other magical & engineering luminaries.
The defense monuments are technomagical artifacts from the former Windhaven civilisation. Three types of artifact exists; the shrines, rings and obelisks. There are many thousands of these across all the major islands in the nothern hemisphere. These structures are about 20 metres tall and appear as carved granite. They have an intricate metal and crystal structure inside. The shrines charge up bird-form shifters with energy as mentioned above, the obelisks and rings seemed to do nothing prior to the storyline. The characters discover that by flying through a ring and then under an obelisk, the two become connected and the obelisk fires automatically at extradimensional creatures that come within its range (of several kilometres). They can fire a continous particle beam in the 100 megawatt class (2 second burst every 12 seconds), guided projectiles that fly at high subsonic speeds and explode with enough force to destroy several close-packed enemies, starburst projectiles that split into forty small highly maneuverable bolts with the force of a light mortar, or draining rays that neutralise incoming fireballs and plasma bolts. Obviously everyone and their enchanted dog has tried to reverse engineer the shrines and find out what the obelisks & rings are for
with very little success prior to the story.
(will post more details if there is interest)
Bonus RAR question : what ability set and bird species would you want if you were a shifter / magic-user in this setting?