Yeah I'm not a big fan of how that worked out... I think mixing the Tiers "in-between" the basic levels, rather than starting after 20, was kind of a mess.
But I do think the idea of shifting to a new kind of advancement, rather than adding levels to the same classes, is a solid one. 20th level is the peak of mortal achievement... above that you start becoming something else (hero-deity/demigod/Initiate Immortal, archmage/avangion/Athasian dragon, Athasian "character elemental"/primordial, archfey/Athasian spirit of the land, etc...)
Have 4-5 different "base" epic paths based on the different character archetypes -- Proxy (or Exarch, Saint, Prophet, etc...) for divine types
Archmage for arcane types
Archfey/Spirit of the Land for "nature" types
Living Shadow(???) for rogue types
Deathless Warrior(???) for martial types
Then alter or add more to fit specific settings (Athas would have Athasian Dragon and Avangion rather than a single Archmage path, and Elemental rather than Proxy for clerical types, and so on...)
Something like a "Kaisharga" path would be good too -- Athasian Liches, but for any class, not just arcane casters -- I don't see why the idea wouldn't be applicable to other worlds. There are already Death Knights (fallen Paladins) and Mummy Lords (Clerics)...