I'm doubling down on Sharlayan. But I'd like point out that they can encompass an entire expansion, to all the naysayers that say its but an island.
Sharlayan dungeons and Eureka have proven that the laws that govern reality are immutable to them. Anti-tower was a fantastical journey that starts by walking through a door in Matoya's cave. Gubal Library seems to forgo the rules of gravity and have ethereal stair cases. Eureka has changed the rules of weather generation, the follow of water, directions like north, south, east, west have all changed direction.
We can go to the Sharlayan island, but I'd posit that the island is just for the city and from that city we can go to places that exist but would never be on a map. Among places that these paths could lead are to the aetherial sea (where Hydaelyn and her crystal are), to the real Amarout ruins on the source, to another Ancient location hidden on the source, to any type of research facility in both technology and magic, to a safe house that contains the means to bring mortals to the moon, and to a location to test our resolve as heroes where we fight ourselves.
My point is 6.0 could be a fantastical journey beyond the idea of grounded game play. These places could be colorful and magical and weird. Remember we are talking about a civilization potentially linked to the Ancients (Hydaelyn faction) that have something to hide.