As I said before the work involved in creating a PC feat/subclass/replacement ability/etc.... is so much higher then doing it for an NPC. Like
@Remathilis said take Legendary Actions, I have a master swordsman who has honed their craft to amazing levels they are super quick/strong and I give them LAs like extra attacks and movement to highlight that amazing quickness. A PC wants to train with said NPC to gain the same LAs how are you handling it? Because sure in fiction it's something that is teachable, but in practice we can't be giving PCs legendary actions without completely warping the game.