Re: Another Faction(s) besides Rebel and Imperial...
Let's also remember that we don't even need new species to introduce new factions. There are smugglers, pirates, resistance forces and rioters, mercenaries, XWA-style private shipping companies with their own security forces, the list goes on and on. Project Reality has implemented assymetrical NATO vs. insurgency and Conventional Army vs Militia battles, and they're very fun and interesting to play. Imagine a SWAT-style scrap where a ragtag force of rioters or terrorists or paramilitary group or what have you, fight Imperial stormtroopers, who could employ tactics like door breaching, flashbangs, fast-roping onto roofs, what have you. Or a space map where Rebels or Imperials fight pirates or some other faction with its own ships, voice-overs, faction name, emblem and playing style.
As was said above, there is just so much material to draw from.
--the awesomeness that is Boatmurdered.