tldr version: Not here, a million times no. A related site if you must, but please not here.
I have never seen a community-run site able to post consistently interesting challenges that didn't devolve into a mutual-congratulation society, a firehose of vaguely-related submissions where few people ever look past the first dozen (or what's next to their own), and sometimes both. It's actually one of my hallmarks for "sites I probably won't like."
Successful executions of this sort of thing all have one thing in common: a small group of people (often one) with a consistent goal and the critical skills to evaluate success. The best systems have that person actively curating the interesting submissions, and often submissions are invisible to participants.
This is largely incompatible with the way SE is set up. It's difficult to be consistent by committee, and it's largely incompatible with users/participants voting on submissions. The largely subjective and de facto competitive nature (as soon as people con vote, it's competitive) creates conflict.
Voting also brings the inevitable Velvet Elvis Effect: popular is not the same as good. HDR, much maligned, is a great example: you can do it well, some people do, most do not. But the awful, horrible HDR is just as popular, or more, as the well-done versions. I don't think that's fair to the people who do good work, nor to the people fooled into thinking that "it must be good, look at all those votes."
Another aspect of successful executions is that they mirror other aspects of the site: challenges/assignments meant to instruct are best on sites dedicated to instruction. The best art-oriented projects come from art-oriented blogs. Strobist does strobist stuff (it's basically its own word now). People seek out those places for their focus; it's not something we have here, or are even interested in.
I think it's also not entirely fair to users to do poorly. Look around at other examples, and you can see that most participants are low-to-mid-experience photographers with no particular preferred working method. They usually have a mild technical inclination. This is a broad community, certainly, but it's still a bit of an echo chamber; you see the people who participate, you don't see the people who don't.