> - Side-scrolling shooters or action games, platformers

Have you played the DOOMSDAY scenarios in OC? They were basically auto-scrolling maps where you have to be fast enough to keep up with the scrolling (similar to some levels in Super Mario). For those, I built a bit of a system with trigger objects for events (like meteorites, changing scroll speed, enemy spawns, halt-until-enemies-killed, audio effects, etc.). I think with the new editor and proper props, it would be possible to create a small pack that makes building quick side-scrolling parkours in co-op in the editor pretty easy.

The only disadvantage is that they don't have very high replay value. So you would have to rely on people to build a lot of them :) (e.g. in a contest)