I have never tested GM multiplayer. I am able to implement it but it probably isn't very efficient (however, according to documentation, should avoid packet loss). Things I can't make are some fancy visual effects, such as blur or distortions. Also liquids, that is, their physics. And Worms-alike destructive scenery, probably. Atm I don't remember anything else.

Unless someone is up for drawing the whole levels, I suppose we'll stick to tiles, so I'd like to hear your opinions on player mask size and tile size. For comparison, in JJ2 it's respectively 24x24 and 32x32 (duh). We could consider tile sizes of 24x24 up to 64x64 and mask sizes of 24x24 and higher. Big tiles reduce level designing freedom but improve efficiency and, usually, quality. A big mask means big sprites, so it improves quality, but sprites take a lot more time and effort to make. Also, the bigger the mask is, the less you can see beside it.

Oh, and compare Sean's idea to Jumper, although maybe it's just me who sees a lot of similarities.