Dead Rising 2 The idea is there...
The idea and gameplay is there for sure. Everything about it is smooth and very well polished. That said not much happens to keep the player interested. Everything you currently have is very good though.
Some examples would be
1) Add a story type mode or levels so the player has motivation to continue through, sure achievements are nice but an actual level system would be better. Each level would be more difficult.
2) Add a variety in the zombies. Seeing the same zombie over and over can get repetitive, so why not add some different ones , like some blue, some with a weapon etc. And take it a step further, as you progress through the levels, introduce "Harder" zombies, such as one who is faster than normal, one who does more damage etc. That will introduce even more strategy to your game.
3) Add some rpg elements to get the player addicted. For example after every level you can upgrade your weapon or buy a new one etc, this will add to the addictiveness of the game.
These are just some ideas to progress your already very solid and fluid game.
Like I said earlier, nothing technically is wrong with it, its just that it needs more content =)