I have a sinking feeling Hammerfell is next. Next most likely is Valenwood or Elseweyr are tied for second most likely. Finally there is some peripheral evidence to suggest Black Marsh, but it's not much.
Edit
Hmm... I have been inside my chamber of understanding and I think I have narrowed down the candidates for next game.
Game Development Reasoning:
They will want to save some human provinces to space the series out a little.
They also Need something significantly different from Skyrim's cold climates and... for good measure Oblivion (let the feelings fade a little).
Hammerfell is very different, but it's also way too obvious. We had some very obvious candidates in the past and they have never made it.
Summerset Isle is not quite different from Oblivion in climate and it's still just a little too soon to go back to an environment like that. So in this regard it is a bad candidate.
Summerset is a good candidate because it offers us new architecture/elves to break up the pace. It might not have a radically different climate, but hey it's new and exotic while still being tiny bit familiar.
Summerset is also kind of obvious a next step. So I think we can reasonably expect to not have this next.
Summerset is a fan favourite.
Elsewyr is also significantly different different, very different compared to what we are used to.
Valenwood is also radically different from our usual. It also has familiar elves as oppose dot beast races.
Black Marsh is again very different, perhaps a little too different at this time.
Black Marsh is a fan favourite.
High Rock could be seen as a little too familiar with the whole mountain thing going on, and we've kinda sorta been there. Not really. I think it is really unlikely.
1-5 my odds of these being in based on this evidence. 1 meaning almost no chance in hell, and 5 meaning you can bet your life on it.
Hammerfell 1
Summerset 3
Elsewyr 2
Valenwood 3
Black Marsh 3
High Rock 1
Lore Reasons:
Hammerfell has lots of reasons too, tons of reasons. So many reasons you can make a book out of them. The whole fighting to keep both parties out is one aspect. Them fighting the AD. The registered DLC thing. It's even more obvious of a plot hook than anything. Unfortunately obvious bets have proven to be usually wrong in these sorts of things. I'll be surprised if we get this in our next game. Not to mention from a lore perspective as to the big chess game being played, I can't see this being a big location.
Summerset Isle has tons of connection to this place, and it involves an ancient plot to destroy the towers because they do not like gods or something. My lore is a little rusty, my point is they have a big part in the plot, but it's too early in the play to be here. It's got tons and tons of prominence and connection, and I feel it's too obvious with the Aldmeri Dominion. Tower Destroyed.
Elsewyr has some pretty good connections with the AD starting to throw their influence in here. Unfortunately this "influence" doesn't seem to be all that much. We haven't seen all that much of their effect here and it doesn't really seem like there is all that much going on here yet. They did trick the locals with that whole moon business. Ultimately it seems to have ended uneventfully. No tower. There is the possibility of a "New Mane" to remake a tower there.
Valenwood seems to be another fairly likely candidate form a lore perspective. They joined the AD and now might regret it with that whole cleansing business. Has a tower... dun dun dun..... it's quite specifically stated we don't know if it is active or not. It "rooted" itself. Most mysterious. We would need to see the state of this tower for sure. Also given what we know of the war. It's a very strong candidate in this aspect.
Black Marsh might have one of the strongest, but most subtle lore connections. It is hypothesized by me and a few people that there is another of "The Towers" hidden deep within the swamps being protected by the Hist/Argonian. The way they reacted during the Oblivion crisis was too well timed, and with such ferocity and skill to simply be protecting themselves. It almost seemed to be a divine mandate to them. We have to wonder how the hell Argonians can do this (hist gives them powers). With the Red Mountain explosion it could have triggered the Hist unintentionally to attack the country in self defense mode. If we don't go here next game it will still be a little ways off. It's all coming down to Bethesda and how they want to do the order of the towers.
High Rock also has a tower near it. Not much other current lore to go there though. However, if something were to happen to the hypothesized tower in Black Marsh then we would be setting up for a return to here. What could be the last of these towers. Depending on how Bethesda goes at this, we could see a return here, perhaps not now.
1-5 same scoring system. 1 = Almost no supporting lore/plot to go here. 5 = Tons of supporting plot to go here, or lots of lore.
Hammerfell 1
Summerset 2
Elsewyr 3
Valenwood 4
Black Marsh 3
High Rock 2
Averaging out.
Hammerfell 1
Summerset 2.5
Elsewyr 2.5
Valenwood 3.5
Black Marsh 3
High Rock 1.5
Most likely candidate is Valenwood followed by Black Marsh and then Summerset Isle.
Take your bets. Long odds welcome folks.