The Scenario:
That rat bastard Q Junior is up to his old tricks. Apparently Janeway's lessons in the Delta Quadrant didn't take, and he has now screwed up reality so bad that the Borg have been replaced by the Xenon from the X-Universe (as of X3: Terran Conflict), around the time of TNG: "The Best of Both Worlds". (Yes, there was time travel involved. And for continuity's sake, "Q Who?" took place with a Xenon capital ship instead of a Borg cube.) Q Senior turned Junior into an amoeba for his troubles
, but then decided to wait and see what would happen instead of fixing things right away.So the Xenon are now at the doorstep of the Federation's core sectors. What happens?
History: The Xenon were originally the Terraformers, created by Earth's government in the early 22nd century to seek out worlds and terraform them to Class M specs. At first they worked quite well, and Earth built a respectable little empire in the Milky Way. But in the early 2140s, a faulty programming patch made the Terraformers go haywire and begin terraforming humanity. By 2146, they had destroyed every colony save one (Aldrin had not applied the patch) and were attacking Earth in force.
The Terraformers were ejected from the Sol System by means of a massive deception on the part of the Terran military: A battle group led by Admiral Nathan R. Gunne lured the Terraformers through Sol's only jumpgate, which was then destroyed behind them, trapping both fleets in the X-galaxy. The survivors of the Terran fleet eventually settled worlds and became the modern-day Argon Federation. The Terraformers disappeared for a while, then reappeared about 400 years later as the Xenon. By the present day (2938 AD), the Commonwealth (the major governments of the X-Universe) has fought several full-scale wars against them.
As of X3TC, they're enemies to everyone in the X-Universe except the Split, with whom they have established an uneasy truce based mainly on MAD.
Capabilities: The handy thing about using a race from the X-Universe in a versus debate is that their offensive and defensive capabilities are well-defined: the game gives us numbers in actual metric units. Individual weapon yields are measured in kilojoules (although the actual numbers are anywhere from single-digit MJ for the weakest scoutship-grade laser, to single-digit GJ for the mightiest battleship cannons). Shield and armor energy dissipation is also measured in MJ, from single digits for M5 scouts, all the way up to 12 GJ for the most powerful M2 battleship. Recharge rates are given in hundreds to thousands of MW for both lasers and shields.
For numbers, we'll assume that all Borg shipbuilding facilities were replaced by Xenon shipyards, all tactical cubes were replaced with Xenon K, standard cubes with Xenon J, and spheres with Xenon Q. Links lead to pages on the X3 Wiki containing their full stats.
FTL in the X-Universe amounts to teleportation, being essentially instantaneous regardless of distance. The limiting factors are energy storage, that it takes ten seconds to charge the jumpdrive, and that you can't just jump randomly: you have to have a specific destination. (No "warp strafing" or blind jumps are possible.) Normally jumpdrives require a jumpgate on the far end of the jump, but the Xenon have developed the ability to jump a beacon to specified coordinates and jump to it. Such jumps require a great deal of power, however, and only their corvettes and capital ships can generate enough.
Goals: Due to the faulty programming that created them, the Xenon attempt to "terraform" all biological life out of existence. However, their self-preservation programming allows for such oddities as their ceasefire with the Split.
Tactics: Unimaginative, and possibly where the Xenon are at their most Borg-like. Xenon attacks tend toward raids by small parties of fighters, P's, and Q's, in which they more or less fly straight to the target and blast anything that gets too close. The Xenon heavy capitals, J and K, rarely leave Xenon space.
Unlike the Borg, they do not assimilate organics, but will salvage and reverse-engineer military technologies such as weapons. (Xenon warships have been seen mounting both Commonwealth and Terran equipment.) Xenon ships' interior defenses respond immediately and ferociously to boarding actions. (Capturing a Xenon capital ship is possible, but it requires large numbers of elite marines. Starfleet security guys would probably be massacred.)