yeah, the super cool version of this would have a rating system and some sort of list of levels people have made.
Maybe Steam marketplace supports this kind of thing? Needs investigation.
Could also string some levels together to form a Challenge, rather than just playing individual ones. Maybe let you control what upgrades are available or pre-selected at the start of it.
I think there's a ton of potential here to get a lot of cool content in circulation, even if half the user-generated levels will look like penises. :)