We have no strong reason whatsoever to go to Meracydia or Sharlayan as there has been no indication that really, not to mention Sharlayan is too small for anything beyond a Hub.
Meanwhile, we've been thrown constant references to Garlemard, it's civil war and the forces at play there. We even get 3 instances in its outer reaches: Terncliff, Gangos and Bozjan Southern Front.
Events at 5.4 and the 5.5 might lead to bring the set up the expansion to have an additional twist to go to Garlemard, other than going there to clean up the remains of an Empire that is already falling.
My current theory would be that Fandaniel plan only has effect in Ilsabard (it could always have the risk of spreading over to the rest of the world), destroying Garlemard and other countries. The result would be a hellscape continent riddled with the same creatures we meet in Emet-Selch recreation of the Final Days along with survivors fighting for both survival and dominion over the remnants of the Garlean empire.
I think the Moon could make for a good Raid or final area (a la Azys Lla) but not as an entire's expansions continent. Cecil and Golbez have potential to get a cameo as Garlean officers if the Moon were to be a raid.