it'd be very cool to see Guns of Icarus become the first none Valve game to allow player made content added to the game
More like the 25th (Civ V, Skyrim, Gmod, etc), but no matter.
I believe Muse has expressed interest in doing so in the future, but that was quite a while ago. I'm not sure if they'd be able to implement something like that with all the work they need to put into Adventure Mode.
But who knows, maybe it'll happen one day.