Oooh, there are loads left who would improve the series. We've not seen series stalwarts Inca or Maya yet, nor the Ottomans. I'd wager there will be another African civ, either Ethiopia/Mali/Zulu or possibly some one new like Ghana. Always the possibility for Carthage/Phoenicia too. You still have civs like Celts (who have been in multiple games) another ancient ME civ like Babylon/Assyria/Hittites or Byzantium and another mercantile proto-Italian civ like Venice/Genoa ect.
Lots of places they can go, it'll be a case of thinking something unique about them like the Cree trading route but I'd wager we're pretty much guaranteed to see the Ottomans plus another South American civ, another European civ and an African one.