There is a new map with a string " Warning: This must be flagged as "in development" until 10.1" titled [Dragon Isles Underground] just like how we went to Argus in legion or the lands of the first ones in shadowlands.
I knew 10.1 will take place below Azeroth the first time I laid eyes upon the new datamined underground mushroom tree models.
With how Blizzard is trying to go for a more classic DnD xpack now, the underdark isn't too big of a reach. Notice how they named MoP as an inspiration for Dragonflight, shamelessly skipping the obvious Cataclysm references just because MoP had better reception from fans. I know Blizzard inside-out, Cataclysm 2.0 is shaping up too look nice.
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10.0 will be Dragons in a n open-world dragon isles theme
10.1 will be Dungeons, in an open-world, very spacious albeit underground collection of zones akin to Deepholm
Dungeons n' Dragons