The first Dead Rising hit pretty close to perfect for me.
But I guess if I could design a game about zombies, it'd be similar to Saints Row 2 in it's size. The city would be littered with zombies and you'd have to go out into the sea of undead for missions like scavving for food and ammo, helping people in danger etc. You'd also have the ability to fortify your own personal stronghold and over time you could increase the amount of real estate under your control, building up area by barricading it and pushing back the horde.
Done right I think a game like that could really be fun.