Personally I would've liked to see one on the First. Two random ideas I had:

1: Ronka. A set of shifting ruins, designed to test heirs to the Ronkan Empire, similar to Heaven-on-High, and to hide their arcane secrets. Lots of stone monsters and golems like those inhabiting the first section of the Qitana Ravel or the solo instance with the three Viis.

2: The Empty. Gated behind the Eden raids, a series of tunnels have been uncovered by Ryne and Gaia's restoration efforts, leading to a warped labyrinth full of Light-corrupted beasts and leftover sin eaters whose presence threatens to undo all of their hard work.