I'm thinking it's more and more likely that we don't see Garlemald proper in 5.0. By the end of 5.x, we'll have made peace with at least one major faction in the Empire, and that will serve as the excuse for us to be able to operate relatively freely in Garlean territory, with the 6.0 city being Garlemald itself. The story will probably be focused on a full-blown Garlean civil war, and wrap up any of the loose ends from the 5.x storyline.

Once 6.0 comes out, it'll be 2021, over 10 years after Yoshida started on FF14, and 8 years after ARR's release. He's said in the past that he wants to do his own game someday, and IIRC he mentioned 10 years as a good amount of time to devote to FF14. I wouldn't be surprised if he's planning for 6.x to fully wrap up the story arc that started with 1.0, and pass off FF14 to someone else. By that point, we will have seen all of the three continents, and future expansions can be stand-alone stories that involve exploring the rest of the world (Meracydia, the New World, Old Sharlayan).