Recently I discovered Heroes of the Mythic Age and Small but Vicious Dog. It got me thinking, could one devise a retro-clone hack that generally kept the characters at a low power level by changing character advancement from a traditional level based system, to some sort of skill based system. As an example, currently when a character levels they get a new hit die or extra hit points, better saving throws, better attack bonus, new spells and class abilities, and more proficiencies etc. Instead of this system where the character improves in a variety of ways all at once, it would allow the players to spend xp to improve one of those items, which is similar to how WFRP handles character advancement.
While thinking about this I realized that trying to balance each improvement would be a lot of work. Then a thought popped in my head. "Hey, 3e has these things called feats that are all supposed to be roughly equal, you could just use those". I think this is one of those ideas best left unexplored.