Talking about co-op, I'm not sure if it's technically possible but It would be cool to have not just the other player voice coming via chat but also the sound effects relative to the action he is involved with when not in the same area as yours.
Remember the movie Aliens? The marines chatter mixed with gunshots and aliens' screams when the shit hits the fan? How cool would it be for one player to actually hear environment screams and such coming from the other player session via chat channel? Something like this:
Player 1: "I'm close to the power generator. Get ready to call the elevator."
Player 2: "Ok, be quick tho. Something is not right here."
Player 1: "No worries. I'm almost there"
Player 2: "Oh shit!"
Player 1: "What's up?"
Player 2: RATATATATATA! AAAARHGH!
Player 1: "Hey! What's going on?"
Player 2: RATATATATATA! BOOM! "They are coming out from the fucking walls!" AAARGH! RATATATATA! "Oh my god! They are EVERYWHERE!!" BOOOM! RATATATATA!
Player 1: Hang on!
etc
Add to that
- voice chat coming from main speakers when players are close to each other (in the same room)
- a PIP showing black and white low res video feed from other player 1st person view.
And you get the perfect horror sci-fi co-op experience.