Damnit I had a nice reply typed up to you guys but I lost it. GRRRRRRRR
Just some (mostly human) enemy suggestions for prospective alien plays, I will try to re-create the detail again later:
- Locals (badly-armed, numerous civvies, to fight in Harvest and Research, angry civvies in terror missions, etc)
- Inquisitors (crazy anti-alien cultists, want to destroy aliens and their tech)
- Men in Blue (police forces and investigators - noisey Mulder and Scully types - working for X-COM/Governments. A bit weak on weapons, but good on numbers. Eventually go full SWAT because screw you. Also, their "terror" unit could be Attack Dogs!)
- Military (soldier types, complete with tanks, jeeps, the works. Militaries of nations. Lags one tier of weapons behind X-COM)
- Gray Ops (mercs working for unscrupulous corporations, power-mongers, shadowy manipulators, ruthless dictators - EXALTish types - who want to steal the alien tech to become rich/powerful. Had an idea that they could use primitive cyborgs or something unscrupulous)
- Rebel Hybrids (human-sectoid hybrids who rebelled against alien overlords using a mix of alien and human weapons, and psi. Want to overthrown the Sectoids but don't trust pure humans not to kill them. Could be the Alien game's equivalent of Ethereals)
- Enforcer Androids (Enforcer Androids created by X-COM's Professor Able Standard. In this timeline, Enforcer program was not cancelled and sorta the ancestor of Apocalypse's Androids. Tough, strong, no psionic abilities but invulnerable to psi as well. X-COM's mutons, so to say. There's a mod somewhere that adds Enforcers to Open X-COM -
https://www.openxcom.com/mod/x-com-enforcer)
I'm not sure how X-COM itself would deploy. Of course, it should be the strongest force, and constantly evolve. One cool idea would be to get a lot of X-COM vets and have them create "deployment mixes" - think of different numbers of differently-ranked troops with a diverse mix of weapons, strategy and AI. It would be especially interesting if X-COM had access not only to vanilla but the new weapons modded by the community that are often seen in mods - shotgun, assault rifle, flamer, sniper rifle, grenade launcher, etc.
If you're doing something that has the aliens fight other aliens as well (Rebel Aliens/Aliens vassals of Micronoids/TFTD Aliens vs UFO aliens), then it gets easier, but I tried to do my best in thinking. Could even add Apoc aliens if you feel like it.
There's also the issue of ships and alien missions, but I will talk about it later.