This isn't my original idea. The flash version of this game had a custom maps feature where you could create your own maps for team game modes only. These maps also had to be symmetric in order to make it a fair fight. Otherwise, you could design the map so one side has a much better starting position. I say bring this feature back, but make it better. Allow players to design maps for team-based and free-for-all game modes. Also, there should be an option to either turn symmetry on or off. If it's on, and you're designing a map for teams, then teams start on opposite sides of the map in designated positions. This assures a fair fight. It's just like before. However, if symmetry is off, then the spawning positions of all players is completely random. They can spawn anywhere on the map. This also makes it fair, even if you get spawned in a crappy position. This happens on the default maps too where you can spawn in a bad position.
For this game I don't think it'll play out very well. Considering you can make flat maps and very easy maps, the XP multiplier will be really shit most likely, something like x0.5. Custom maps also makes other existing maps completely useless because you can make them by yourself. I think it'll be good for a Lv100 unlock with x1.0 multiplier.
Maybe there could be a feature allowing you to place permanent bumpers/portals like those in the campaign.