Looks impossible. Creating your own maps, obviously possible through mission editor and such, but not normally, within a game. Brickbuilt is possible though. Leveling up players would require a lot of different playertypes. Monsters customizable? I think that might be possible unless you're talking about movement speed and health, etc. Of course, you could, for health, create an entirely new variable and be able to modify it within the game to do all sorts of stuff, and use tower bricks that would deal damage to that variable. Still, though, inconvenient and extensive.
