Between Meracydia and Sharlayan, Sharlayan is the far more likely candidate to me.

1. Its msq focus was far greater than Meracydia in HW, Meracydia had Tiamat's dialogue only even the warring triad was not mentioned as Meracydian until side story with Unakalhal whereas Sharlayan had;
-Forbidden spells and spell experimentation (Flow to the now used Teleport)
-Motto (Knowledge seek no man)
-Thier culture (architecture/sentries/government)
-2 dungeons Gubal and more importantly Antitower, I find it very likely that Sharlayans would've built a second tower closer to home perhaps one that runs deeper into the Aetherical sea (probably best way to get to Hydelyn and thus Zodiark because right now, how do we get to the moon? Meracydia would have to have an Allagan rocket or something)
-Master Matoya's crystal eye, I doubt she has the only crystal of light from another era, especially since Krile knew what it was while Y'shtola didn't.
If we add side story than Meracydia has a bigger emphasis but still pails in comparison to Sharlayan as you have to add Ast questline and how ruthless they are about not sharing certain knowledge and Saint.M's dungeon so plant life experimentation vs Meracydia's 2 beast tribes (Trees/centaurs), and the warring Triad.

2. Meracydia's size is as big as the 3 great continents, this fact alone screams that it would take multiple expansions to deal with what Meracydia can offer and I would want that, Meracydia has a lot going for it and I would want it to have the love and care Ishgard and the 1st has had.

3. Sharlayan's size is much smaller and a split expansion focus with it and Garlemald is going to be far less hazardous especially when we have a much more established area compared to Meracydia.

Those are my basic reasonings, I want to go to both Sharlayan and Meracydia, both sound like very interesting places to go, I just think Meracydia would be too disjointed at this current stage or it would suffer the same way Ala Mhigo did. (As I currently think we will get an Emporer primal in Garlemald based on how the Empire is taught to revere him, the civil war going on, the alliance at their doorstep, it's all a recipe to make an primal desperation of the masses, faith, the only thing missing is power source but given that it is Garlemald and their association with the Allagan Empire not going to be too hard to find one I think).