Great idea, in fact, awesome idea. I love alternate history almost as much as I love zombies and post-apocalyptic goodness, so this one definitely appeals to me. I agree about adding in more buildings; what you might do is fill in every block with buildings, but make them a gray just a few shades darker than the background. That way you fill up all that empty space, but don't detract from the important attractions/buildings.

edit: Oh, and I really like the fact that the WIC building is a big black monolith - is that a subtle hint that they're Evil Corporate Overlords?