There'd be zombies, lots of big marine-type fearsome looking dudes, inifinite ammo, co-operative gameplay (with CPU or human allies) in huge areas, where you do things that are actually interesting and make sense, with an underlying theme of playing as a badass shooting things, with the odd other badass shooting things.

Like that movie Dawn Of The Dead (remake) mixed with Star Wars Battlefront with a fair bit of Resident Evil, and maybe some (a lot) Castlevania overtones to the whole affair. RE4 with allies I suppose.