I'd think that at best, it might be better served as a Chronicles of a New Era esque sidestory quest chain than its own expansion, especially considering the theme of shadowbringers. As bar a Stormblood scenario they likely aren't gonna try putting whatever plotline they have planned on pause for this kind of story as a MSQ chain.

That said I do think if such a premise were to happen I can see both Mikoto and the Pixies being relevant given that theformer's a memory expert and the latter had been established to to have interactions with dreams.

I can also see this being the expac where the WoL-as-Azem would fight the archtypes of which the current Primals originate, such as Ifrita.

EDIT: missed the bit about it being post Zodiark, in which case it might be possible, and it might also triple as a prequel per se and a new starting point for adventurers