Aashi (the guy who Fandaniel took over the body of) is a popular meme character in Japan apparently (from what I understand, it has to do with the way he pronounced his name is like the word Oreo? I guess their equivalent of Chicken Tenders Ready to Fry and Big Fat Tacos lol). But ya, I suspect he will get offed at some point before the end of the xpac.
When I was watching Jesse Cox play through, he did bring up the good point that Fandaniel satisfies the "FF games with a crazy, "stage kid" villain like a Kefka or Kuja" that doesn't have a motivation and just wants to watch the world burn like Joker, which the game hadn't really had previously. Not my favorite villain type, but it does at least mix things up from the tragic villains we have had in the past, like Nidhogg, Emet, and Elidibus.
So they did say the story ends in the base xpac story. So 6.1 will be a new story or at least the transition into one.
My theory is that the WoL ends up discovering that they, and perhaps Zenos, are what is causing the end of days 2.0 because they both are more complete and closer to unsundered Ascians. So in order to save the world, we basically do a Hydalean kick 2.0 and split ourselves and Zenos into fragments (and in turn Zodiark and Hydalean as well). Or maybe it also restores the lost worlds that had been rejoined in the past as well? As a final gift from Hydalean, we are able to retake our form, but we lose a lot of our power that we held before that we had gained through gaining a lot of our soul shards.
Maybe not like that exactly, but they said this wraps up that whole storyline, so I suspect whatever they do will basically represent a "next journey" for the WoL and I have a feeling they will end up having us do a hero sacrifice vs. losing anyone else.