I just had an interesting thought. What if Epic play isn't handled by levels above 20, but as a campaign template that affects all levels?
Here's what I mean. Levels 1-20 equal the range of mortal potential. Level 1 is a novice and level 20 is a grandmaster. Humans, by themselves, don't get better than this.
But let's say you want to play a game where the characters are divinely blessed, or demigods, or fueled by the blood of demons. They would still pick a class and start at level 1, but they would also gain (for lack of a better term) an epic destiny. As they gained levels, they would get abilities and bonuses from both their class and their destiny.
This better matches most epic fantasy novels and ancient hero myths. Hercules was a demigod from the beginning. Achilles gained his immortality from being dipped in the River Styx.
Design it right, and the players could obtain, or lose, an epic destiny any time the DM wanted to change focus.
This also solves the problem of few people ever playing high level games. As a separate module applied at first level, far more people would be interested in playing it. It might even help address some of the divide between the tastes of AD&D players and 4E players.