9. Car customisation
While playing through GTA 4 we never really felt attached to our cars. If we'd had the option to completely customise them, from performance to decals, we might have felt a stronger bond. Imagine having tools similar to those found in Microsoft's Forza 2, effectively letting you apply any paint job you'd like. Head to the car performance store and pick up some nitros, a few neon lights and you're set. It'd be great in single-player, but it would be brilliant in multiplayer games, with each gang having their own custom designs.