I would love to see someone create co-op games for smartphones. Imagine sitting around and playing as one of the door gunners on a B17 type game together with friends. Heck each of the positions on the bomber (gunner, bomber, pilot) could be either a human or computer and folks could join or leave at random. How about a space simulator where each person performed a key role, like weapons, propulsion, engineering, etc? That would be a very fun experience indeed.
This would probably only be possible on a lan given the latency issues with wan networks, especially cellular.