I have been reading this thread and I have a couple of quesitons.
If a character could qualify for an epic feat, can they take it? I am leaning towards no because they are not 21st level characters. I just wasn't sure if you were treating the feats gained after 6th level as being epic level feats and so you could take one if you met the prerequisites other than the 21st level one of course?
For example, Chaotic Rage, requires Rage 5/day and chaotic alignment. You could easily get the five times per day by taking the Extra Rage feat.
Also, I have an idea, but I don't know if it twists from the path you are intending to travel, so if it is not of interest, don't worry. What if you moved characters attribute adjustment from every fourth level, to every third level and then an additional increase every 15,000 XP after 6th level. Or to slow it down, every 30,000 XP after 6th level.