One possibility I've been considering lately is that we're still within a stable time loop. That, by the end of Endwalker, the Source will be in such horrid condition that the only way to save it from annihilation is to deliberately trigger the Eighth Umbral Calamity via unleashing some heretofore unseen stock of Black Rose. The WoL will survive, but be thrown forward in time over two hundred years, winding up in the Eighth Era shortly after Syrcus Tower was sent back in time.
The three biggest obstacles to that idea, afaik, are as follows:
1. Even if there was some way to keep all the old world content accessible, perhaps through a Zidormi-esque character or device, I doubt the developers would be willing to do this. This is probably the biggest obstacle.
2. We came back to the Source and presumably shared our exploits on the First with enough people that they couldn't reasonably forget it after Black Rose-ing the world half to death. Especially since Cid and Nero both survived Black Rose according to one of the Tales from the Shadows short stories. On the other hand, maybe the entire reason they began working on the possibility of time travel is because they knew it would be perfected by their successors in the future. And now I have a headache. I hate time travel.
3. Even discounting the above, we also woke up G'raha Tia, took him out of the tower, and dumped the Exarch's soul and memories into him. A lot would have to happen to him to justify shoving him back into the tower alone and without the Exarch's memories.
So, overall... I doubt what I suggested would ever happen. But it is a thought I keep returning to.
