Let's presume for a second that this is a trilogy. I know its not confirmed but I think it's a likely scenario that they'll try to turn this into a trilogy, given theyve compared this to the XIII trilogy. And I think this project will wear out its welcome if it goes beyond three parts, to be completely honest.
That means that the second game would likely end at either the City of The Ancients or The Northern Crater.
Assuming City of the Ancients, That would mean that the game has to cover this much ground:
Kalm/Nibelheim Flashback - Mythril Mine- Junon and Junon Boat Sequence - Costa Del Sol - Mt Corel - Gold Saucer/Prison - Cosmo Canyon - Nibelheim - Rocket Town - Return to Gold Saucer for Keystone - Temple of the Ancients - Bone Village - City of the Ancients.
You may be able to trim down some of those, but you'll probably have to find a place to put Wutai in there too. Point is that this is a lot of ground to cover, especially with the production values they're going for. I just don't see them covering that much ground while also developing a massive, detailed open world. XV had an open world as a selling point, and its story and structure suffered heavily for it.
If we compare this to XII, which uses a segmented, zoned world, I think it's more comparable (using IGN's walkthrough as a reference):
Nalbina Fortress Intro - Rabanastre - Giza Plains - Rabanastre Palace - Bhujerba - Lhusu Mines - Leviathan - Sand Sea - Tomb of the Raithwall - Golmore Jungle - Eruyt Village - A Long-Ass trek to Archades - Archade - Draklor Lab - The Feywood - The Ridorana Cataract - Pharos Sirius - Finale
