I think the next Elder Scrolls should take place in Akavir, seeing how all the other provinces in Tamriel are being covered by ESO.
Not all of them are in great scope though.
Only a few provinces are covered near enough fully.
Orsinium isn't featured, despite technically being in a region of High Rock that is featured.
Most of High Rock is covered, bar the border to Skyrim.
A tiny portion of Hammerfell is covered. Only the northern Alik'r Desert is touched, bordering on the Illiac Bay.
Summerset Isle only covers the (comparatively) small Auridon isle.
Almost all of Valenwood is covered (possibly all, depending on the map used to determine borders)
A smidge of Elsweyr is covered, with most of it being a single small training island. (Probably the least featured province)
Only a small section of northern Black Marsh, and the Black Marsh/Morrowind border is covered.
A few small areas of shore-line near the sea that divides Vvardenfell from mainland Morrowind is covered.
Only Eastmarch (Windhelm) and The Rift (Riften) from Skyrim are covered.
Only the very central area of Cyrodiil is included, including Cheydinhal, Chorrol and Bruma.
It doesn't include Kvatch (I think), Skingrad or Anvil, so none of the south-west.
And it doesn't include Bravil and Leyawiin, so none of the south-east, nor the Niben.
Really, there's plenty of area of Tamriel untouched by TESO or any of TES games, particularly Elsweyr and Black Marsh.
TESO covers a lot more land than almost all other TES games, but it doesn't cover entire provinces, and hardly touches some of them.