Has anyone tried spiral knights? Its free on steam, and pretty fun. Although the level design is complicated. The world changes based on what people do in it.
Basically there are 5 crystals. And gates, controlled by them. Players go into the gates, kinda like elevators, and bring back crystals. Then people can put them into gates that aren't open, and influence what the theme of the gate will be.
Its a lot more complicated than that, actually. too hard to explain.
But the game is still really fun.