I probably prefer a more 4e approach with epic destinies.Actually, with the epic boon system neither your class nor your character level advance. You instead advance through feats, boons, and ability scores. We’ve played with a few time and i personally enjoyed it more than levels. In fact, we plan to do a campaign it starts at level 20 we liked it so much.
For 5e, the boons need to be fluid, allowing the play free choice of which ones to take. Some boons are stupendously better than other boons. Players will and should cherry pick the better boons.
That said, the boons can organize thematically. Some themes will relate to Psion becoming an immortal mind (reminds me of Ob1). Some like Wizard becoming an immortal archwizard. Some themes wont relate to class, such as an undead Lich or so on.
At minimum, each theme should be levels 21 to 24, a full "immortal" tier. But higher levels are also possible. Even so, the player can pick any boon they want at each level, so the immortal levels are fluid.
Simply becoming level 21 should get immortality for free, but the flavor of the epic destiny will determine HOW the character became immortal (magic, ascendant, undead, etcetera) − in addition to the first choice of boon at level 21.