Although I am not a fan of 4E in general, I think 4E has the right idea of what to do with epic levels - integrate them into the system sufficiently that no sudden 'break' is necessary like in 3E (so, for example, no disjointed epic level spellcasting system with no connection to the system in the 'standard' levels). Of course, this approach would be facilitated if you the design of epic levels into account when designing the 'standard levels' in the core book. I really hope you are taking it into account.
Agreed. In fact, I'd almost like to say that levels 15–20 are "EPIC" and leave it at that, but since the concept of higher than 20th level play's already pretty much built-in to the game, that's not really a good option. WotC's choice to extend the range to 30th and call 21st to 30th is an elegant solution.
The problem with epic level play as it stands, as far as I see it, is that there's no end point. The open-endedness is attractive on one level (No limits! Play the same character forever! No hard-wired end point to your advancement!), but it's also a huge disadvantage, since without an upper limit, there's no way to set a good scale. There's always SOMETHING BIGGER. I hit this problem more often than I wanted as regards feedback to the Demonomicon articles and Fiendish Codex I; no matter what CR one sets a demon lord at, it's too high for some folk and two low for others. Since you only have a starting point and not an ending point, it's VERY difficult to design anything for epic level. What's the difference between a CR 23 and 29 and 33 and 59 and 3492 creature? How do deities fit onto that scale? And if deitys DON'T fit onto that scale, then how tough does a character have to be to fight a deity? What CR is the most powerful creature in the Multiverse?
When Paizo does an Epic Level supplement for the PF RPG, my preference would be to basically present epic level play as its own game, similar to how the Immortals set worked. Once you go beyond 20th level, you start a new game in a lot of ways. You might still be "21st level" but the game assumes that's the baseline and goes from there (which argues pretty strongly to reset the "level" to 1). Of course, if we do a new game for Epic Level, I'd want to keep things transferrable between that game and the PF RPG, so that one could use a low level epic monster as a high level PF RPG monster after a hopefully simple conversion process.
At the very least, I'll be pushing to have our answer for epic rules to be a closed scale with a level cap. Open ended level caps don't work.