To start with, I think it's worth saying a few important things:
1. 7.3 is slated to be the end of Dawntrail's MSQ arc; Endwalker was unique in that the arc ended with 6.0 because it was a wrap for the MSQ arc that began in A Realm Reborn (or maybe even Legacy), so 6.X was just filler to get Dawntrail prepped and ready to go (not unlike 2.1 - 2.3 being filler for Heavensward). 8.0's MSQ arc should start being set up in 7.4.
2. The Scions already took a major backseat in 7.0 to Wuk Lamat. Complaints about the latter aside, I'm not sure how popular pulling them out of the spotlight has been. Also we need NPCs to run Trusts, and they're already there...
3. The whole point of sending us to the other side of the world seems to have been an effort for a fresh start. As such, I do not think we're going to go back and explore "old world" areas except as side content (Stellar Exploration, Deep Dungeons, Custom Deliveries, etc.). The MSQ writers clearly want to break away from preexisting lore and do their own thing; I can't fault them for that, though the results so far have been less than impressive.
The current trend seems to be setting up a reflection as a bizarro incarnation of a previous Final Fantasy title and us stepping in to resolve some crisis (after the fact). 6.X was the first taste, Tuliyollal seems to continue this (Gulool Ja Ja, Zoraal Ja, and Gurfurlur are all named after XI characters with characterizations contrary to their original incarnations), and Alexandria cements it. While they may pivot considering how poorly Dawntrail's been received (all you hear in news about it these days is how stale the game is and how subs are dropping like flies), the most likely path forward with what we know is using the Interdimensional Key to visit other reflections which will have similar bizarro setups to IV and the Thirteenth and IX and the Ninth. Where they go from here is anyone's guess, but if I had to venture a guess I'd say something to do with X (since it's sandwiched between the games Dawntrail references and one of the most popular games besides).
Will Preservation and Calyx have anything to do with it? Maybe. Will we go around eating Azem shards? Probably not. Will real Sphene join us on such adventures? It's a personal want, but who knows.
Apropos of Azem reflections, it's deliberately left vague as to whether or not the "original" Golbez or the RPR Avatar is "Thirteenth Azem." Yes, I know the EE suggests it's the latter, but there's too many suggestions in the game to rule out the former. It's a moot point regardless - if it's the former there's no way to actually retrieve them, since "original" Golbez's memoria crystal was lost or subsumed into Zeromus (whose own memoria crystal is being used to revitalize the First's Empty), and if it's the latter RPR doesn't function in-lore anymore.