Shadow of the Colossus 2?
Kotaku has an interesting and unfounded rumor about Shadow of the Colossus 2 for the PS3. If this team created a new game in the same style with a co-op mode, I think that it would be pretty fooking awesome. If I let my imagination run free for a moment, I can see the same kind of grandeur of the original with a cooperative puzzle solving slant to it. Requiring the 2 players to be simultaneously figuring out how to help the other to take down the boss could be a ton of fun. I envision hooting and hollering as each player gets closer to the weak point and screams of defeat as one finally loses their grip and falls, only to try the ascent again. I think this would be very enjoyable.