FFXVI and XIV connection theory

[Cleretic]

Though fan service alone doesn't explain why we see Fran twice in the series as two entirely separate people.

...Yes it does.

We see that several times over in this game: Tenzen (from XI), Krile and Galuf (from V), Doga, Unei and Xande (from III), Matoya (from I). Hell, literally right now in the current story we have Golbez and the Fiends from IV, and in the exact story you're describing we have Ba'gamnan, Ramza, Agrias, Mustadio, and Cidolfus. And that's not getting into the ridiculously long list of returning named and figures across the series that keep turning up ONLY because of the fanservice.

Sometimes, there's not a lore reason for a thing: sometimes it's just that Biggs and Wedge are fun go-to names, or that the story that they're currently telling could use a Matoya.