...what game would you create?

I think id create a free roam game like GTA, but with zombies everywhere, and make a huuge map, like 10 times bigger than GTA San Andreas.
A bit like Prototype, but instead of you being a freak person with crazy powers, your just a normal civilian trying to survive the zombie apocalypse.
Everything would be interactive, for example you could go into any building in the map.

I think that would be a seriously awesome game, doubt we have the engine to handle it yet though.