I've mentioned this before, but during my E6 game, I've allowed certain Epic feats to be taken as capstone feats. Mostly because they don't great the game at all. For example, Combat Archery, which allows someone to fire a bow without provoking an AoO. The ranger/sorcerer/wizard 1 spell Arrow Mind does that as a swift action for a number of minutes, and there are feats that either add to your AC vs AoO with a bow, or don't provoke an AoO if it's an exotic ranged weapon. So it only makes sense to do that.
While going through the feats, I came up with a capstone feat of my own. It's a variant on an epic feat.
Energy Resistance [Capstone]
Benefit
Choose a type of energy (acid, cold, electricity, fire, or sonic). You gain resistance 2 to that type of energy, or your existing resistance to that type of energy increases by one category: 2, 5, 10, 20. This feat does not stack with energy resistance granted by magic items or nonpermanent magical effects.
You can gain this feat multiple times.