They'd have to find some way to make the instances fit in the lore of Eorzea. The Caverns of Time worked because they could refer back to actual events and historical moments in Warcraft history.

The easiest way I could see this working, with as limited lore implementation as possible, would be for the Wandering Minstrel to conjure images of classic FF fights as "legendary stories" that such bards are wont to tell. That way, they don't need to have actually happened or even be based in Eorzea's history. Plus, we already have the assumption that the Wandering Minstrel is powerful enough to conjure things into material being, such that Ultima (Hard) and Savage Coil actually exist in a tangible way.

I'd like for SE to dig deeper and find a way to make revisiting classic FF stories work in an actual time travel or "dimension travel" way, but the Wandering Minstrel would be an easy shortcut.