Given the world-changing political events that characterize the 4th era it only seems natural that the next elder scrolls adventure needs to take place in the Summerset Isles. Now that Alduin is dealt with the Empire/Thalmor conflict is the most pressing concern, and given the fact that an agent independent of the Thalmor is responsible for preventing the end of times I'd say that moral is at an all time high for a counter-incursion into Summerset territory. It need not even take the form of a military campaign however, I'm sure internal intrigue within Summerset is more than sufficient to provide ample plot points even without an Imperial invasion. Now, one might argue that the Thalmor is far too powerful to be contested at this point in TES history, but given their recent defeat in Hammerfall I would say that they are similarly weakened to the empire. All it would take it one decisive victory in Elswyr or Valenwood (and as we've seen in Skyrim there certainly are bosmer that resent the Thalmor) to be well on the way to taking the war to the elves.

On a broader historical scale one could even think that the ass-whooping the Empire received at the hands of the Thalmor was a good thing to turn back the increasingly bureaucratic and political trend and set the Empire back on the track to uniting all of Tamriel. Elven aggression was actually the necessary precursor to arouse the fervor for the final unification of Tamriel. The fat imperial politicians would have been perfectly happy with its current holdings, but destiny demands that Alessia and Talos' project meets its penultimate conclusion. This need not be related to any doctrine of man's superiority over mer, after all loyal elves have a long history as imperial agents; it is necessary however that a little human toleration characterizes the onset of the first Tamrielic empire, but once a thoroughly cosmopolitan mindset has been established throughout the empire a meritocracy will probably win the day, occupied by man and mer alike.

Moving away from the historical anolysis and looking outside of the games as a series it seems that Summerset is a prime candidate stylistically. I hope we can all agree that we want to move far beyond the bland medievalism of Oblivion's Cyrodiil (what happened to Cyrodiil being a rain forest?) and continue on the path of wondrous fantasy established especially by Morrowind and carried on somewhat faithfully in Skyrim. The only acceptable candidates I see are Hammerfall, Elswyr, Black Marsh, mainland Morrowind, or Summerset. Hammerfall isn't politically interesting because it's too secure and well off at the moment. I have serious doubts that Bethesda will ever make an elder scrolls game in the beast nations simply because people are anthropocentric and I doubt it would sell well. Morrowind is unlikely because it has already been done, and that leaves us with Summerset.

Practically speaking from a game-making perspective Summerset is perfect too because it is an island. It appears to be about half the size of mainland Skyrim, and as such is feasible to make into an expansion scheduled for early or mid 2013. When I hear expansion, I want something bigger than Bloodmoon, and Summerset provides the perfect opportunity for Bethesda to outdo itself.

Summerset Isles as an expansion? I doubt it. Hell, it's bigger than Vvardenfell (by most maps anyways), and developing that took almost four years. I agree that further development into the Aldmeri Dominion and/or the White-Gold Concordant is logical story wise, but the scale would have to be a bit more manageable. Perhaps another Solstheim sized island could be created (it didn't exist before Bloodmoon) in the Abecean (maybe even using the Island of Stirk).

Now as either a spin-off or developed for TES VI (http://www.gamesas.com/topic/1339291-official-beyond-skyrim-tes-vi-6/), then I'm all over it. I've been waiting to see the Isles for nearly ten years!

Well, it is clear at end of civil war in Smyrim, that Thalamor are next foes.
But to set next game in Summerset Isles would be weird,since the next focus will be race war, that would mean that Empire is back on its feet and has managed to even butcher its way through Valenwood and Elswyr, and I doubt that empire could do that at this moment.
Although if they manage to summon all the men to fight against Aldmeri Dominion and forge pact with argonians, they could destroy Thalamor in both Valen and Els.
I have high hopes for Valendwood and Elswyr as places where great men vs mer war will be fought.

It would be difficult, but I feel it would be possible within two years, given that they already have the core game complete and they're sitting on way more money than they were back in 1999. Another option to consider is the fact that the summerset isles for the most part consists of a main island and a sister island, a campaign on the smaller island would be possible.

Another thing is that it seems the TES games are taking an epochal turn, I have a feeling the next full game might be fifth era. I don't mind this though, because they really want to make sure you feel like a rare hero. I figure this whole White-Gold concordant business might need to be completed in this expansion set, if not in one maybe in two or three.

Why not Valenwood? It's a land of weird bugs, cannibals, gigantic trees in jungles and forests.

A lot of people on this forum believed Summerset would be the setting for TES IV (Myself included). We were wrong then and will probably be now. Bethesda has a habit of choosing things that are unexpected. The only expansion I ever played that was anticipated in location was Tribunal.

So I have no idea what's going to happen next, but I wouldn't think you have a good chance of knowing before anything is announced..

I am having some gut feeling that Betsheda is going to skip this whole mer vs man war alltogether and put us somewhere xxx years ahead, again.

I remember back in 2009, people strongly believed that Elder Scrolls V was going to be in the Summerset Isles, but there were also those who saw that Bethesda trademarked "Skyrim" and here we are two years later with TES V: Skyrim. Bethesda has apparently trademarked "Hammerfell", so if we were to follow the previous example, we might very well see TES VI: Hammerfell. Having said that however, Bethesda has been known to surprise, so it's really anyone's guess.

Source?

I don't think the op means as an expansion but as the next series.

I don't think any TES expansion will ever take us to a region which is the other side of Tamriel to the one the present game is set in. Maybe an island of off the coast or a small chunk of the neighbouring region but nothing more than that.

IMO we've seen plenty of northern Tamriel. Some delving to the south would give Bethesda a chance to present a different climate than what we've usually seen and expand on some of the more mysterious cultures and races of Tamriel.

That said, I really think I'd like to see Elsweyr as the setting for the next game, since I don't think our current level of hardware could do justice to a more lush environment like Valenwood or Black Marsh.

We are talking about an expansion unless I have serious issues with my reading comprehension. As a matter of fact since this is Skyrim specific and we're not discussing the next installment (which we have one official thread for), I'll ask a moderator to move this.

Discuss the TES future here; http://www.gamesas.com/topic/1339291-official-beyond-skyrim-tes-vi-6/

Also, Bethesda already said there won't be expansions, but instead a bunch of DLCs (or mini-expansions if you will) like with Fallout 3.