I feel like they want to keep folks interested in the endgame, so it doesn't make a ton of sense to throw in someone like Nerscylla after folks like Rajang and Safi'jiiva, as deserving as she is to be in the game. That said, I wonder if they'd be crazy enough to do the likes of something more exotic for an encounter, like Dalamadur (he's kind of in the game already, given that most of the Rotten Vale is his remains) or, RNGesus willing, Ahtal-Ka for the folks that think that Shara Ishvalda is an impressive and crazy final boss fight.
...it's so totally going to be more subspecies, variants and returning elders. That said, I do feel a lot better about Gore Magala (and Shagaru, because why not) being able to get in at some point, since it has a unique mechanic that can also bring about a further tier to the endgame if they so choose, and it's not like the skeleton isn't already in the game.