Yeah, my idea for the sequel would be a huge city, where you start smack in the center.
Then it would be like the Infinity mode in Dead Rising, where the supplies are only usable once. And you have to survive and do as many of the missions/saving people as you can before you decide to make it to the outskirts of the city to safety.
So if you wanted you could just start and run to the edge of the city and beat the game with that ending. Or you could try and see how long you can survive on the cities resources before you think it's too overwhelming and escape.
All the while there are a bunch of story based missions and random saving people, all of which you can chose to do or not to do. And the zombies start in slow/small numbers but as the days go on they get more and more vicious and many.
That's what I would like. Keep with the free roam zombie killing fun that the first game had, but make it really feel like a "What would you do if a zombie outbreak happened and you were stuck in a big city?"