So it's safe to assume that both Nazjatar and maybe Nyalotha will be future areas in BfA either as zones or raids. That being said, what other established locations on Azeroth remain to be used as future content as either an expansion continent or just patch content.
Off the top of my head I can think of Tel Abim, Undermine, The Dragon Isles, Azjol-Nerub, Plunder Isle, Ogrezonia, Goria (Southwest continent in Draenor), Farahlon, the remnants of Ka'resh.
Or perhaps Blizzard will just ass pull some new continent or turn us to space police and explore space.