On the subject of timelines, convergences, and the plot threads therein, I had a bit of a shower thought this morning in regards to the Exarch's timeline. There are four factors to my hypothesis. Spoilers, natch.
1. The Exarch either awoke, or was sent back through time and space, two hundred years after the Eighth Umbral Calamity. I might be wrong on that number, so feel free to correct me if I am.
2. Emet-Selch seems convinced that the eighth Rejoining will free Zodiark and allow him to fulfill his goal of sacrificing the life on the Source to bring back the sacrificed Amaurotine people.
3. Despite the above, in the occasions where it is brought up, there is no apparent Ascian or Zodiark presence in the Exarch's timeline. Combined with 2, this presents something of a puzzle. Where the heck are they?
4. In 5.2, it is confirmed that Elidibus sacrificed himself to serve as the heart of Zodiark. Despite this, there is an Ascian calling himself Elidibus. Urianger suspects the title changed hands, but the unspoken suggestion here seems to be that Elibidus continued to serve his duty even after his sacrifice.
So, that hypothesis:
The reason Zodiark and the Ascians have no apparent presence in the Exarch's timeline is because there was no cross-world soul summoning business to interfere with the Scions' work during the war with Garlemald. As such, we were able to deploy the means to kill Elidibus during our confrontation in the Ghimlyt Dark and actually succeeded. And without even realizing it, we had dealt a mortal blow to Zodiark in the process. Thus, when the Eighth Umbral Calamity rolled around, Zodiark was already dead.
Regardless of how well my hypothesis pans out, as I'm not entirely convinced we'll ever touch on the Exarch's timeline again, I find myself a little more interested in seeing where 5.3 goes with Elidibus's plot now that I've had this thought.