So I've played Skyrim as much as I could and I'm now looking forward to another Elder Scrolls game. That being said, I've only played Skyrim and ESO. I tried Oblivion, but I'm hopelessly superficial and the graphics made me cringe. Anyway, I've had a while to think about some ideas for TES6 and I'm going to dump them here for input, criticism, etc. I like to think I know the lore pretty well (I read the Imperial Library religiously), but if any of these TES6 theories contradict canon lore, please be harsh.
1. The Elder Scrolls VI: Hammerfell
I know there has already been a game set in Hammerfell titled Redguard, but it was a spinoff. This time, the story centers around the struggle between free Hammerfell and the Thalmor as a follow up to TES5 and we get to see the aftermath of the Great War. Who wouldn't like to hunt down our favorite elven racists? However, their has to be a bigger picture than a Thalmor invasion, so I thought about the Dwemer. According to Lore, clan Rourken separated from mainstream Dwemer after the formation of the First Council, migrating to Hammerfell and disappearing with the rest of their kin following the Battle of Red Mountain. Maybe the Rourken clan could be spared the fate of the other Dwemer for separating from their eastern cousins, allowing them to be pardoned by Azura (or Lorkhan) and return to Tamriel. Looking at their exodus in response to an alliance with the Chimer and their battle with Shalidor, it can be inferred that they were very xenophobic and militaristic. This would make them perfect antagonists.
2. The Elder Scrolls VI: Elsweyr
Around the beginning of the fourth era, the moons Masser and Secunda disappeared, causing the Khajiit to cave like the French. Then, the Thalmor show up out of the blue and say they used "ancient moon magic" to bring the moons back. So, I'm thinking that the moons could disappear again and the Thalmor are powerless to act, causing their Khajiit subjects to become disillusioned and revolt. Seeing how the Khajiit were made the way they are by Azura, the main questline could involve Azura as the one who put out the moons for some unknown purpose. Also, a conflict between the two kingdoms of Elsweyr could provide a third conflict.
3. The Elder Scrolls VI: Argonia
Yes, yes, I know it's probably a hoax, but that doesn't mean it's impossible. For this one, I imagine the main storyline being an Akaviri invasion of Tamriel based in Argonia. After all, the Tsaeci of Akavir are said to be reptilian in nature and the humid, treacherous environment of Blackmarsh prevents most non-reptilian life from thriving, making the province an ideal base of operations for the invaders. At the same time, their could be an underground political struggle occurring between the King of Argonia (backed by his Shadowscale guards) and the An-Xileel (backed by the Hist). I thought about this because, after the Oblivion crisis, the An-Xileel rose to power with the help of the Hist who had warned them of the coming Daedric invasion. They soon go on to conquer large amounts of land in Morrowind and accumulate a large amount of support from the rest of Argonia. On the other hand, the King of Argonia is largely uninvolved, hinting that he may not agree with the An-Xileel, viewing them as extremists. So, the player could choose to join one faction or the other in the same way one could choose Imperial or Stormcloak in TES5. It also wouldn't hurt if a little bit of Morrowind was thrown in to the North so that we could see how well the Dunmer are recovering.
4. The Elder Scrolls VI: Summerset Isles
A lot of people are opposed to this idea because no one wants to play in Thalmor ground zero. However, the player could be helping the remnants of the royal families who were supposedly slaughtered when the Thalmor took power post-Oblivion Crisis. As the main conflict, the Thalmor could be trying to rebuild the "Brass Tower" (Numidium) in order to wreak vengeance on the empire for past grievances *cough* Tiber Septim *cough*. What they don't know is that the Numidium still possesses a part of the Heart of Lorkhan (Elf-hater #1) and that it could destroy the isles. So, the player could dismantle the Thalmor and see the other side of High Elven culture that was overshadowed by the negative image presented by the Thalmor in TES5. Plus, the Summerset Isles is in close proximity to the Isle of Artaeum, making an encounter with Psijic monks an interesting possibility.
5. The Elder Scrolls VI: Valenwood
My personal favorite, Valenwood is a really untouched place with a lot of unexplored secrets. In this instance, we could have a power struggle occurring between the Thalmor, who insist on their form of worship, and the Bosmer who revere Y'ffre and try to keep his Green Pact. It was mentioned in the past that a prophet known as the Precursor gained popularity in the province and claimed Y'ffre would soon reveal himself to his followers once more and usher the wood elves into a golden age. This could leave behind a fanatical cult in his wake that is trying to start another Wild Hunt, threatening Tamriel at large.
Food for thought. Go nuts.