Where do you want to visit next?
The Ruby Isles - Also called Imperial Island, City Isle, or the Eight Islands, this includes the island immediately surrounding Imperial City & a few small adjacent islands. With Cyrodiil at war and the Imperial City under siege, this little slice of forgotten territory is perfectly positioned to serve as a future Event Zone with its own dynamics, or as a direct expansion of Imperial City with the same Tel Var PvP dynamic.
Soltheim - We've been there before, but let's go back again! Perhaps we can get a Werebear skill style.
Central Skyrim - Fill in the rest of that gap! College of Winterhold + the Pale. Maybe some expansion of Frost elf lore.
Shivering Isles - The realms of Mania and Dementia offer the perfect place for PvP Battle Royale, each with their own uniquely maddening rulesets.
Hammerfell - The perfect zone to release in conjunction with ES6. Expect heavy lore cross-over, maybe even character cross-overs.
Soul Cairn - We could learn more about the Ideal Masters, and maybe pick up some new Soul Magic.
Black Marsh - Lots of space left here: Soulrest, Helstrom, Thorn, Gideon, Archon. Up to South Deshaan.
Central Elsweyr - The Tenmar Forest between Northern and Southern Elsweyr. Potential home of some endangered beast races.
Falinesti - This one is tricky, but not impossible. Since the city has been taken by someone (Rajhin, Molag Bal, Mephala, who knows?) we could find it in Coldharbour or some other realm. Our completion of the storyline would simply ensure the city's "future return" to Nirn, simplifying implementation.