Next DLC?
Elsweyr. That is one reagion we have the least for.
And then... imperial remnant regions. Nibenay, perhaps.
And I for one can -never- have enough dunmer regions, I really would love to see Necrom in ESO...
More alik'r would also be interesting... what else might be buried beneath those sands?
And of course, there is the western half of skyrim...
And then the rest of Tamriel packaged in DLC form... possibly with some new guilds thrown in as well like TG or DB...

The next chapter... that one I would prefer to see outside the Tamriel mainland. Daedric planes. Far off islands. Etc. Stuff that is seperate in some way...

I really would love to see something on the Akaviri someday... for example, they -could- make an expansion covering the islands between Tamrial and Akavir - there are three of them, Esroniet, Yneslea and Cathnoquey, each reputedly somewhere between the size of a starter island and standard region, and they would almost certainly have been the staging area for the akaviri invasion ten years before the events of ESO, and thus be under akaviri rule at this time... and so might make prime expansion territory, without deadric princes for a change, just find out the story behind that invasion, and prevent the next one they are prepping for over there?

I also would love to see more or the maormer, like a "high seas" themed expansion that covers a lot of small islands between Summerset and Pyandonea - possihbly even with some unheard of forays into the latter, though that might be iffy lore-wise, soncidering just how xenophiobic and isulationist the maromer are... still, it could be possible some daedric prince threaten them to such an extent that they -have- to set this aside and get help from some of the landwalkers... and there could always be maormer-controlled islands between the mainland and pyandonea, to keep the mystery of latter while still showing off maormer culture - just be prepared for -way- more hostility and arrogance then you ever met in summerset!

Other prime options for expansions would be expeditions to "lost" lands... visit the remains of sunken yokuda, see the frozen lands of atmore, search for the lost lands of aldmeris... or perhaps even a incustion to the sload kingdon of Thras, pay them back for the trouble they made in Summerset...