I just read the first couple pages, and I think the theory behind this E6 idea. So I had some thoughts (perhaps others had already mentioned them)...
1) I've been playing with max hit points per level for some time now. I wonder if E6 would be unbalanced if I did the same?
2) Perhaps a choice at each level after 6th. At each 5,000xp after 6th level, you can choose one of the following:
(a) A bonus feat you meet the prereqs for
(b) +1 BAB
(c) 1 Hit Dice woth of hit points (in my case, it would be max hit points)
(d) Gain class abilities as if you gained an additional level in a class. In other words, if you were a 6th level Rogue, and you selected this option, you would get the class abilities of a 7th level Rogue (+4d6 sneak attack), but nothing else.
Thoughts?
A question though... What happens to skills? These don't seem to improve beyond 6th level. Is that a problem?