I posted this in another thread, but it is my unfounded speulation that I have gleened:

"Lots of interesting possibilities. The expansions have had similarities to the main titles; Stormblood vaguely resembled FFII (War with the Empire, nothing too deep), Shadowbringers had several elements from FFIII (Crystal Tower, Warriors of Darkness, Light and Darkness are neither good nor evil, Eternal Wind), Endwalker FFIV (Too many to list, lol). Assuming this trend continues, Dawntrail should have FFV references and story beats. We already have Krile stepping up to be a more important role, her namesake being from FFV. The main story element that I think they'd use is two worlds once separated remerging into one, but of course that's nothing new for FFXIV. Another game that has a major story element of two worlds merging is FFIX. I'm thinking that's the Solution Nine reference, and could be another plan put in place by the Ascians. Perhaps the whole northern continent is from another shard, or another shard is bleeding over into the source and perhaps that's why there's a huge gash in the world, and also why there are two very different civilizations in the same place. The whole "Soul Supply" thing has me thinking about Zidane's people, the Genomes, who are vessels waiting to be filled with souls."