But making such a key part of PC build subject to GM decision-making (on a conventional assumption about how gets to introduce loot into the fiction) is pretty controversial!People got burned out on prestige classes because they became just another level of build complexity. They need to be returned to their original idea of being "narrative loot". You don't choose to become an Initiate of the Sevenfold Veil, you find a very rare grimoire in an ancient ruin that gives you the keys to pursue that path.
4e D&D made boons and training analogous to magic items, rather than to levels or feats, to manage this problem.