That's kind of a cool idea. I did wonder why they specifically had the, "Far Edge of Fate" line in the trailer.

The thing is though, while this could be possible, and I wouldn't put it past SE... I think it's rather unlikely for a few reasons.

One. The circumstances of the cocoon spells are different. When Louisoix attempted the spell on Bahamut, it instantly failed, meaning it was never actually completed to take Louisoix's life. The other difference is that Louisoix's doings were aided by all the Scions praying at the altars, along with an unbroken Tupsimati. Papalymo had only his aether, and the aether he could gather in those few moments with broken Tupsimati.

Two. If they bring him back fully in 6.0, it will detract from the other stories we have to go with, as the cast for FFXIV is pretty dang large now. While it'd be a nice shocker, I don't think there's much Papalymo could add. I don't think they could pull it off satisfactorily.

Three. In the Stormblood end credits, there is one final picture of Yda and Papalymo cheering alongside an Ala Mhigan flag. I think this is the memorial for him, because there is just a lot they have to and want to show off, and they just haven't found the time in game for him yet. I don't think they will, either.