Short Answer: The stuff in TR_Mainland is all done, the stuff in TR_Preview is in various states of completion.

Longer Answer: The Telvanni areas to the east of Vvardenfell are finished, complete with quests and NPCs. The Indoril areas to the southeast (Necrom area) are mostly done if not all the way done; the rest of the Indoril lands and the Hlaalu lands to the south are almost finished with most interiors, exteriors, and some NPCs. The Redoran lands to the west have their exteriors and some interiors done. The Dres lands to far south are just getting started and are only a heightmap at this time.