I'm in the midst of creating a lighter d20 based system that ditches ability scores (for mods only), skills (for ability checks), hps (for status levels similar to M&M), and (of all things) levels. (Technically classes are so abstracted as to not really exist too, but they are still referenced.) The core combat system is incompatible with the standard d20 combat system but it is also one of the more easily replaced systems in the game. Basically, everything is a feat, a creature's effectiveness is equal to its number of feats. When you "level up" you get a feat. While I call them feats, they are a bit stronger in power than normal d20 feats. The magic system is also feat based with a DC system to cast a spell rather than spell slots.