It would be entirely possible to throw a bit of XI in here, in multiple ways. One of course is a couple zones, maybe something akin to WotG XI expac, we see some of the world but not all of it. Another would be more nostalgia but still a possibility, to make a type of port of FF:Grandmasters as an arcade game to Gold Saucer. I don't see all of Vanadiel coming back even if they considered it, but its story is certainly not "dead."

Another chapter could easily open for it, and even the producer letter from last night hinted that it may not be over (could either be for XI proper or speaking technically of continuing the story on the mobile port someday, not grandmasters.) If they did throw XI areas into XIV though it would have to play by XIV's ruleset, which may not be all that bad but unsure if it would be what most people are looking for.