I like the 5e approach to Epic, where each level gains a boon, an Epic tier feat, instead of class features. It maintains continuity with the earlier character concept, and the customizability of feat selection can heighten aspects of this concept.
The 2014 DMs Guide Epic tier needs to develop further, but the approach is sound.
At level 21, the character is by definition Immortal. There needs to be a way for the player to choose the method of immortality, maybe like choosing a background so as to allow customizing a character concept. From 4e inspiration, default immortalities include: Archfey, Darklord (Shadowfell), Archlich, Primordial (Primal Nature Being), Archmage, Luminary (Psionic Cosmic Soul), Saint (or Tsadik or Bodhisattva), Demigod, Archcelestial, Archfiend, Goo (Aberration Great Old One), and something for a cyborg Construct.
There are many ways to live forever and wield cosmic power. Players can pick a favorite.
Normally, Celestials and sometimes Fiends are referred to as "Immortals". But there are many ways to become an Immortal. For clarification, those of the Astral plane can be called "Astrals" or Archastrals, including Celestials and Fiends, as well as the other stuff like Dominions floating in the Astral Sea, and whatever the Neutral aligned Immortals should be.
One gains a Domain, somewhere in the multiverse, such as a Fey Domain, Shadow Domain, or Astral Dominion. (Maybe refer to Dominions as "Domains" as well.)
And each of these are a higher rank of Immortals:
Levels 21 thru 24, are Proficiency +7.
Levels 25 thru 28, are Proficiency +8.
Levels 29 thru 32, are Proficiency +9.
Etcetera.
Inevitably, Epic characters include bigger numbers. But the important part is what they can do. Hit points matter less than the Immortal power to reform ones avatar after it gets destroyed. At level 21, the capacity to exist via an avatar and form more than one, defines what an "Immortal" is.
Thematic powers are more important than ever. This is the "portfolio" sotospeak. A theme of Plant, can divinate thru and transport to anywhere in the multiverse where plants exist.
Boons can grant a thematic a thematic and dramatic at-will power.
For comparison for how to stat an Immortal avatar: here is part of the stablock of Lolth an Archfiend who has officially measurable powers and surprises. Note the feature, Discorperation.
(The statblock doesnt mention the Proficiency Bonus, but it should to get a clear quick-glance sense of its tier. Cha 30 is +10, Expertise is double proficiency, whence Persuasion +22 −10, the ½, would only be Proficiency +6. But it should be at least Pro +7, whence Persuasion +24.)
View attachment 355493