Basically what the title says.
As there are numerous multiplayer-specific ideas I came up with one which heavily favors the SP maps.
I've seen lots of SP maps trying to make moving airplanes / space ships using doors just to end up with a floating nonsense. If you don't believe me, I'll PM you a few IDs even from good mapmakers.
If certain backgrounds were able to 'move' in the same way as doors, the game would be significantly improved aesthetically. Just imagine the elevators from SP maps; it'd look a lot better to have a different BG inside and outside it.
This could be implemented by creating a new 'Tool' in the Level Editor whose name indicates its function.
Any thoughts?