As others have stated, I think you need to start with a vision and archetypes of what the end result is. One of the the failings of 4E epic level characters was that epic levels just became a power boost, but didn't really change the feel of the characters, although that was partly a structural issue as well.
So if I have a fighter, he's really good at what he does at 20th level. Perhaps the goal should be Thor or Superman. Recognizable still as a fighter/bruiser type character but with abilities unique to that character. While you might have to constrain their powers to a structure similar to spells, I would avoid just giving them spells. Thor is not just a fighter that can cast the lightning bolt spell, his abilities are far more powerful. A barbarian may pick up abilities similar to The Hulk, etc.
You could also base things off of more legendary heroic figures, but a 20th level fighter is already getting close to Beowulf's level of power, not to mention people may scratch their heads and go "Bayo-who?"
So start with the vision, then build some rules around that. Superheroes (especially the overpowered ones) are effectively the demi-gods of myth so that's where I would start. Post some ideas of what you think a 30th level warrior, cleric or wizard should be able to do and get feedback on feel. Then work backwards.
