-4player co-op
-a less buggy melee system, the chainsaw is cool but incredibly buggy and seemingly random who wins in a chainsaw fight. gun butting with the other weapons is almost useless.
-When you throw a grenade holding down the L-Trigger it should look at the final resting place instead of the peak of the throw. More often then not I can't even see where it's going to land, I end up just blindly throwing it into the sky and hoping for the best.
-the ability to switch which shoulder the camera sits on. I like the 3rd person view but sometimes the character obstructs what I'm trying to see and not being able to switch which side the camera rests on is rather annoying.
-there's no reason you shouldn't be able to revive yourself instead of waiting for your teammates (like by tapping A), similarly there's no reason the CPU teammates can't revive you.
-use a different button for running... seriously I'm tired of getting ready to run only to jump behind some cover on the wrong side and get wasted.
-let me crouch without being attached to some cover
-offer some more multiplayer modes, maybe something where players can re-spawn, especially if you're going to try stupid achievements like 10,000 kills