Well, I have to admit that ZombiU is probably one of the best games I've played this year, a new genre on itself I would say, a First Person Survival Horror.
No, it's not a shooter, even if you're lucky and find someone else's zombie full of ammo, the game obligates you to analyse the situation and to think well what has to be done before doing it.
For it's sequel, I don't want a "better fighting system" because this game is not about fighting, is about thinking and it doesn't need a combo system or zombies that require you to perform some "just frames defenses" or "special attacks" to kill them. It's all about thinking and knowing where to shot, and where to fight.