This is what I hate about not making these games self-contained. You almost always end up with one of two scenarios:
1. The game ends either in a cliffhanger, or with a substantial amount of loose plot threads in order to drive interest in the sequel.
2. The game comes to a complete conclusion, and the sequel generates some forced conflict in order to build another story.
Sequels to FF games have typically followed the latter approach (X-2 and XIII-2), since they weren't originally planned when the first games were written. XV's story was also written a while ago before they had any sequel intentions, but they could intentionally sever the story since they've decided on sequels so far before release.
I really wish they would just completely conclude this story in one game. I know they want to reuse assets and gameplay systems in future games so they can lower their development costs, but in that case I would just prefer to see them tell an entirely separate story set in that world, with a completely new cast, etc. They can still reuse the environment assets, combat system, etc. Perhaps it could even be a story that was happening concurrently with the events of the first game, which would be pretty interesting. Unfortunately, I'm 99% sure we're going to see one of the two approaches I outlined above
.