Hey, who here would totally play a video game about bike couriers?
I figure it'd be a perfect fit: you'd get goal-based and time-limited missions, sandboxy urban exploration, easily parceled story progression (via things like recruiting and competing with other riders), good hooks for multiplayer (especially with anti-games like competing for most horrifying wipe-out), ridiculous trick runs, going really really fast, and an overall patina of style and awesome.
Think Jet Set Radio meets GTA beats up Paperboy.