Anyway, there was some discussion of an alien mode of some sorts. A certain Wolf89 or somesuch had some ideas, but they were mostly "Let's reskin zombie mode and call it alien mode"
Here's my idea:
One player starts out as an alien that can mimic the host perfectly. He looks exactly like whoever he was going to become, and has all of the person's memories. Missions could vary between infecting a specific player, escaping on the shuttle alone, and similar things to traitor.
The alien can infect a helpless (Buckled down, cuffed, unconscious) human by some ability, I dunno, at which point he becomes an alien, identical to the human infected.
Obviously the quarantine No Shuttle No Escape No Mercy rule would be in effect, and the AI would be required to prevent people from leaving the ship.
Where things would really get crazy is when the alien stops appearing as a human. I'm thinking the alien would have an option to instantly gib, revealing the true alien, which would be very dangerous in close quarters, but would need some kind of weakness. Fire seems like an obvious one. If we wanted to go the The Thing route, we could instead allow it to simply mutate into a hideous humanoid form. If we went that way, maybe it could only be killed by fire, and anything else simply gibs it, the gibs having claws and the like and being able to attack. That might be hard to code though.
Some speculative stuff. Obviously the alien would need some kind of special abilities in human mode too, or it'd just be traitor with infection. Maybe it has some kind of electromagnetic sense that allows it to see people through walls. If that's the case, maybe using a taser on it temporarily or permanently destroys that ability.