Me and my friends are just waiting for Voilition to annouce 4 player Coop. If Gears of War 3 can pull it off why can't Saints Row The Third. Make the missions harder and wider and split the objectives so that 2 players can approach from another angle.
I'd almost rather have an online mode that automatically put you into the city with like 12 people or so, who were running around doing their own thing. Kind of compare to an online racing game like Burnout Paradise.