In case you all are unaware, Morrowind was by far the greatest Elder Scrolls game (and is currently my favorite video game of all time, beating out my beloved Ocarina of Time) so if you were going to do a prequel it should be set in Morrowind with Nerevar, Dagoth Ur, Sotha Sil, Almalexia, and Vivec. You can include the dwarves as well as have a big part in their disappearance. It would be familiar but they can completely change the landscape and appearances (make all the Ghostgate citadels as Telvanni towers) and make the area larger. It's like how theoretically Twilight Princess had the same setting as Ocarina of Time but much more fleshed out and larger.