They need to complete Morrowind first. We still haven't been to the mainland. Perhaps they could do the top half of the mainland as an expansion and the rest as an expansion for Blackmarsh.
Vivec felt bigger with it's hiidden underworks, secret basemants and multiple cantons.
Whatever province/daedric realm beth adds I hope that has a big city, because there really wasn't anything like Vivec or The Imperial City in Skyrim (All of the cities were cool and unique though).