It is common speculation that TES VI will in some way, shape, or form, involve the Thalmor or the Redguards considering all of the new lore we got in Skyrim and how well it sets up potential plots. While I don't like or dislike the idea it is all but confirmed to not be the case.
I. First and foremost, Elder Scrolls games do not follow up each other, Morrowind and Oblivion are no different than Skyrim with how easily their predecessor could have followed up on them. Oblivion is most obvious, Bethesda could have chosen any province and write a story about rebuilding from ruin, or the political implications of the country being invaded by Deedra and the intrigue that follows, instead we jumped into a new era and read about the events in books.
II. The second reason is related to Kurt Kuhlmann and his departure from Bethesda in 2023. His aspirations for TES VI's Main Quest would have been very close to what most people, shoutout to Fudgemuppet, have guessed, a proper follow up to what Skyrim built. However, he was not given role as Design Director as he claimed was promised years ago, thus his leaving, and I highly doubt Alan Nanes, now appointed Design Director, would copy what Kuhlmann had in mind.
TLDR: Kurt Kuhlmann wanted to do Thalmor vs Redguard but didn't get the role he wanted so left, Alan Nanes could do anything all we know, except for copy Kuhlmann's idea.
I could be wrong about this, but I am very confident in the reasoning, if I missed something or there is a gap in my logic let me know, and as always, tell me your thoughts, I'm curious.
EDIT: Maybe I didn't make my point clear in the post so I will try again.
TLDR 2!!: The Thalmor PROBABLY will NOT be the main Antagonist of the MAIN QUEST, Of course they will IMPACT the world of TES VI, but the plot lines set up or hinted at in Skyrim will be resolved either in the lore, side content, or not at all, depending on the setting.
YES, the games do have influence I am well aware, Daggerfall gave us the Orcs as a playable race, and the Oblivion crisis screwed everything up, shaking the empire's power in Tamriel, but the MAIN ANTAGONIST or threat has a connection LOOSE at best. Alduin, Mehrunes Dagon, Dagoth Ur, prove my point, unless I have missed or misunderstood something.