We'll probably have a better idea after 6.3, or at least the upcoming Live Letter preview for it on 23/12/22.

Map

You're not going to explore all of Ilsabard in a single expansion, and there's no need to obsessively uncover every part of this map before proceeding. If the Void is going to be the overarching plot thread for this story arc, then the story will likely need to get us to Meracydia within about four to five expansions or so. For practical purposes, I think the next most likely place to build a story will be around Dalmasca and Corvos. We've already met Fran on a few occasions, so a starting point could be wherever Lente's Tears and that mystery 'Ashe' are currently holed up at the moment. One possible starting city for the region could be Valnain, which sits on the map just south of Nagxia (the latter which is the location of the Golmore Jungle, as per the Ivalice questline). There's also an entry in the EE Vol 2 (p.98) on Dalmasca which makes reference to Leá Monde, a city known for its wine (and perhaps demonic Voidsent cults). With the recent remake of Tactics Ogre it's probably a fairly convenient time to explore the region in more detail.

If that's the case, the second city would probably be Locus Amoenus in Corvos. Fourchenault hinted that there were older names for the city during the role quests, so I wouldn't be surprised if one such name is Archadia (given that they are both Classical literature references to 'paradise'.) They've already done this once by linking Rabinastre and Lesalia. There are lots of possible Void-tie ins here, especially with the lore from the Bozja questline and factions like the Sect of Germonique and the Church of Locus.

As for where they go for the subsequent expansion, it's really hard to say. You could continue to explore northward for another expansion, or just head to the South Sea Isles next and come back when it's time to visit Aerslaent and the New World.