Elidibus is gone. Time travel expends a massive amount of Aether, and he himself tells you that creating a conduit to Elpis will drain the last drops of Aether from his soul. Emet describes likewise how Lahabrea's reckless soul jumps left his soul progressively more broken, like his Phoinix, burning itself to ash. I presume this which is why his ARR/Heavensward incarnation is a hollow shell of his former power. Emet is really the only unsundered who could conceivably come back because of how stingy he's been with his bounteous reserves, and he's clearly smart enough to retire when he's ahead. I'm sure that he'll make an obligate cameo again at some point, though. I personally think that he should just possess the Wandering Minstrel and get his revenge through devious mechanics.

As for the events of Ultima Thule, they essentially confirmed the natural laws don't really apply in there, being a world driven by your internal perceptions. I think that you probably could have summoned anyone or anything in there had you the conviction to bring it forth.