I have, and the thing is, with so many members partaking and entering so many different versions of the required content we could cover quite a few different types and styles quite easily. Not only does this boost the content available to the newbies coming to the site looking for tutorials, guides or useable engines, but it may also prove to show our members some types of battle systems, artwork or collision engines that they simply hadn't encountered or thought of before.Those aren't suitable topics for weekly/monthly challenges. Seriously, a sprite tutorial? With all the different styles, genres and techniques out there? That's FAR too general for a short challenge.
But I do think that there is a lot of good that could come out of some weekly challenges, not just for coding but for all Game Developing aspects.
A few examples would be:
Simple Battle Systems
Decent Dialogue Engines
The same applies to battle systems and collision engines. I doubt we could even agree on definitions for those things.
I don't think you've thought about this carefully.
I have thought this through somewhat, I just don't think you're seeing the real potential that something like this could have, not only as a fun event that anybody could jump in on, but also as a way of generating open sourced content and tutorials for new developers.