We’re getting Varian / Sylvanas / Dreadlord-themed armor sets in Guardians of the Dream!
LeystTV created a few shorts previewing all 3 sets that will be available for transmog in Patch 10.2. There are currently no items in the game tied to the appearances, so the sources are unknown.
Dreadlord Transmog Set
Sylvanas Transmog Set
Varian Transmog Set