What time period do you want it to be set in (although I hear that it may be centered around the Cold War)? Do you want it to be a return to the series roots or remain futuristic and modernized? Do you miss that lunge-to-prone move from the first game?
One thing I'd like again is interactive maps. I first experienced this with Killzone 2's DLC that had things which actually affected your match, such as a bomb going off that would kill you if you weren't indoors or a sandstorm obscuring your vision. The next and last time I experienced that was with a Black Ops map which had a rocket lifting off. Nothing big, but at least it was something.
One thing I'd like again is interactive maps. I first experienced this with Killzone 2's DLC that had things which actually affected your match, such as a bomb going off that would kill you if you weren't indoors or a sandstorm obscuring your vision. The next and last time I experienced that was with a Black Ops map which had a rocket lifting off. Nothing big, but at least it was something.