Most of the dungeons, save for the final one, are previewed before the expansion's release. To my memory, anyway. I was aware of A Realm Reborn but didn't follow it in the media, Heavensward was extremely tight-lipped on Azys Lla, Ala Mhigo was kind of a non-brainer with the way Stormblood went, and The Tempest, much less phantom Amaurot and its accompanying dungeon, were also kept very close to the chest.
On that note, though... 6.0 (Endwalker) is supposedly going to have more story than the usual expansion (up though what would usually be the X.3 patch) so I suspect there might be a little more combat content to match - 2 or 3 more dungeons, maybe a trial or two.