i think there are a total of 5 dlcs bethesda planned on releasing for skyrim, at least it is based on oblivion(4 minor dlc, 1 major i think correct me if im wrong. Only had shivering isles and knights)
Since we had dawnguard, hearth and dragonborn, i say maybe 2 more. Or bethesda could make even more. However, we have yet to see a major major dlc. Unless you count dragonborn as that. But IMO it is a semimajor dlc.
Redguard is rumoured to be the next dlc(read those posts and news). If thats the case, maybe some cities in hammerfell could be featured?