What I'm presenting here is a first draft / idea set, put up here for feedback.
The basic notion is that, past 20 hit dice, instead of gaining more levels, you start to convert existing levels into gestalt levels
. Of course, there need to be a few modifications...
1) You get full skill points for whatever level you add. (Otherwise, for example, a 20th level rogue would never gain skill points again...)
2) Skill max ranks increases by one per two levels past 20. So 24 at level 22, 25 at level 24, and so on.
3) Cross-class skills no longer cost double.
4) You get one feat every two levels, starting at level 21.
5) Epic level feats are available with GM permission - and may be available even if you don't have the listed pre-requisites (especially in the case of things that were written assuming you gain max skill ranks at one per level...)
6) Inherent equipment needs to progress at some rate. Maybe 20-40k per level?
7) XP costs per level can't continue their astronomical climb, since actual power isn't increasing that much.