SO when I remembered the Myst online game I remembered how fun it was to enter an area with people and just see how far in you can explore.
So we could make a puzzle game, with Box2D or whatever style of game, where you can travel to different areas like an MMORPG. And some areas require you to go in with friends to find everything, and other areas you can get through solo but friends can always join you.
So perhaps like a puzzle game but with a central area where you choose where to go so that it is more structured. I love the idea of us submitting levels somehow, of course once again we may have to make an editor.
A simpler plan that could still be fun is a faster paced game with the option of having PVP fights.