Um, a while back, we set up a "design your own RPG" contest, with a timeline of one month and a few other restrictions. I put one forward called "The Awakening", about a post-apocalyptic Pacific Northwest with psychic powers and whatnot.
The Awakening RPG PDF
Anyways, it was a fun little project, with everyone helping out everyone else's ideas and working within a framework of certain core "rules" (the theme was, I believe, "magic", with sub themes of "ruined cities" and a couple others I cant' recall off the top of my head).
Actually, I wouldn't mind doing something like that again. It was a lot of fun.
My game was based around the idea that levelling up characters is kind of boring, and is often about what is most powerful or tactically wise for your character, as opposed to any sort of organic PC growth. So, in my game, you didn't level up outside of play - you levelled up during play, by using your skills. If you didn't use first aid, it didn't improve. And, as skills levelled, they activated character bonuses (like "feats", I guess). There were a lot of problems, but I don't think anyone ever released a "full" RPG - and besides, are any RPGs ever really "complete"?
Still, it was a fun process, and I recommend everyone give it a try at least once.