Meracydia seems most likely to be used for MSQ content since it's so well known in the lore, and is currently quite mysterious. Thavnair trades with them, Tiamat might want to go back, and I think Riol says he wants to go etc. So it's a good place to 'Start Over' as an adventurer if we need to.

Sacred Sites + Forgotten People + Isles makes me think Spyra type content, but maybe thats because I've wanted an FFX type raid in the same vein as Ivalice for a while.

I feel like the New World will mostly stay as "That place over there" for a while, but if it is ever explored I cant see a continent that big being side content.