Co-op. I can't even begin to describe to you how much I would absolutely adore it if the game was fully co-op. 2-4 players preferable, but 2 is just fine. Imagine being able to have a chase scene where you're not fighting the controls by having to drive and shoot at the same time.
I don't really care if they even implement it into the story, having another dude with you, in this huge open-world game, would be absolutely fantastic. I really can't find any faults with this idea.