The trouble with doing stuff that is gameplay-affecting, is that it rewards great players by giving them even more awesome tools. Which breaks the rest of the game for them, because not only did they have to go through the 9th circle of hell to get the stuff, but now anything less than the 9th circle of hell is even
easier based on the loot they brought back. What we ought to be doing is letting the monsters from there loose elsewhere on the world to keep things interesting, heh.
Not that there can't ever be meaningful rewards for doing that sort of thing, but we can just slap uber enchants down there and have it not break the rest of the game for players that get those enchants, unless we also do more stuff to alter the rest of the game. Getting the ultimate weapon that kills the end boss in one or two hits is fun, but that also marks the end of that game. With a world that's supposed to go on forever, that sort of end state is highly undesirable.
But a race of underground-dwelling NPCs is entirely what I mean by a cosmetic reward. You've unlocked some new playable characters, and their stats are a bit different from other NPCs, and they look different and say different things, but they don't actually make the game easier to have around. Cosmetic sounds too shallow, so perhaps cosmetic/thematic/story would be a better way to put it. Non-player-avatar-power-buffing would be another way to put it. Getting new and interesting things to do as a reward, even new spell options, is fine -- so long as those options don't let you overpower the rest of the game.
