Personally I dont really see why we would need rewards for that - if you want to make a personal challenge out of content just go ahead and do that?

For me its actually more fun to just set those challenges for myself and my group - it makes the challenge more fun because we do it only because we want to do it and not because of some shiny reward at the end.
I dont need a reason to create a challenge for myself within the game, the challenge itself and the knowledge that we made it, is reward enough. I dont want challenges be turned into yet another grind.

Certain ideas also sound a bit difficult to track down, just on a technical side - so if they wanted to make more challenging content, I'd actually prefer them to spend that time with creating new content instead of re-visiting the old stuff yet again.