1.) Class-based multiplayer only. Team Fortress with Aliens and Marines. (No predators, sorry. Too unbalanced.) Always struck me as a no-brainer.
2.) Tactical turn-based strategy game. Like X-Com, but viewed from a Lt.'s command chair position in the APC or whathaveyou.
3.) Co-op survival a la L4D1/L4D2.
Here's another one. How about an Amnesia style Alien game?
For those of you who don't know about Amnesia: The Dark Decent
(BUY IT!!!), it's a first person shooter game
without the shooting. There is not one weapon that you can pick up to fight anything. It's just you, your mind and your awareness.
Check out this video and you'll see what I mean.
I know playing as a character with Amnesia is about as common as a realistic military game, but it's one of those elements that has always existed in the Alien franchise that no one ever seemed to care about after the first film. Like in Amnesia, you play a character who wakes up in some abandoned complex or ship. If your an alien fan, you probably already know why your character has lost a good chunk of their recent memories. The game can incorporate elements such as racing against the clock, taking care of yourself to ensure that (when you figure it out) you don't accellerate the condition that's going to kill you, and just try to figure out what's going on.
And since there is no combat involved, imagine what the designers can do with the actual alien. They can have it stalk you, run up an air vent and come out at another one, hide in the dark, hide in a pile of debris ect. All kinds of stuff. And what about face huggers or chest bursters? Imagine hearing those little things slithering and skittering around while you try to find a way out before they nab you. Because remember, you don't have any weapons.
I'd like to see an Alien game done in that direction. More of an atmospheric, suspenseful experience rather than a quick shoot'em up.