I got an interesting idea after reading the Imperium Mafia thread. What if the Mafia doesn't want to kill everyone?
Aliens have invaded your town, in order to accomplish a simple objective. Your goal is to kill all the Aliens before their job is completed. Each Alien have a different mission, but if the Aliens' objectives are the same, they may both win. The goal of the Town is to kill all Aliens, in order to prevent any Alien from winning.
Humans
Townie - Normal citizen. No powers, just your wits and a desire to live
Agent - An FBI agent investigating the situation. Can investigate one person per night to determine if they are Human.
Doctor - Can prevent one person from being killed by anything other then a lynch. Only protects against one attack a night.
War Vet - Can kill one person every night.
Alien
If at least one Alien wins, the entire Town loses. The alien will have 1 of the following goals (not all goals will be used).
*Grab the FBI Agent For Study- Once during the game, the Alien chooses a person at night and takes them onboard their space ship for study. Both vanish. If the person was a FBI Agent, the Alien wins. If not, the Alien loses.
*Survive - The Alien is there for some other purpose, and is simply hiding as a human. Their only goal is to survive until the number of Aliens is equal to the number of Humans. The Alien win as long as he is alive.
*Exterminator - The Alien sees Humanity and the other aliens as a threat to his race, and wishes to erase this threat. The Alien kills one person a night. The Alien wins when he is the only one left alive.
*Lynch - The Alien wants to cause a diplomatic incident in order to get his race riled up to invade Earth. If the Alien gets lynched by the Humans, the Alien wins. This Alien may only post up to a maximum of two times per day.
I hope this is a quick game.