So... if HiGuy and Jeff happen to get bored at some point... I had an idea (probably has been thought of before or something)
Envisioning gems ('Ever'gems). In singleplayer, these would function exactly like normal gems. The difference is purely for multiplayer (sp2mp in particular). Imagine: There are two players and two evergems in an sp2mp level. Player 1 gets both gems, and so player 2 tries to finish - but he can't, because on his screen, the gems didn't despawn when player 1 got them.
Basically, the idea is that each player has to get every gem in a level before finishing. It would make sp2mp seriously awesome and enable gemmed levels.
Having said all this I realize there's probably something in the engine (or SOMETHING) that stops this from being possible. But just going to throw it out there and see what response it gets.