A story would be nice but not needed, or how about the ability to throw things other than explosives. Trash cans, chairs, lamps, and other random stuff. Or how about being able to drive, or fly to different places. Rescue missions, more people,(i got a ton of these) being able to carry more than two guns, flamethrower, swords, axe, being able to fight the zombies (like ufc stlye, striaght beatdown), in general make it kinda like RE5 and Gears meashed together, chainsaw guns(Gears stlye), ninja level(try to bypass zombies without being detected), shots of adrenaline(temporary being super human), charactor design(seems standard), being able to tell cpu what to do, playn as a zombie, multiplayer maps, can play as human or zombie in different games(guardian, capture the flag, teams things like that)
All seems resonable.