There is an almost guarantee the next Expansion will consist of the Thalmor and The Aldmeri Dominion and will be a follow up from the Civil War Questline,
But what I'd like to see is The Moot. Ether Ulfric becoming the HighKing or Elisif becoming the HighQueen and I think you should be able to ether challenge Ulfric for the crown in a combat sword to axe duel or get to know and then marry Elisif so then the crown is handed to you ether way.
So then Skyrim will have a Dragonborn HighKing or HighQueen depending on the sex of your character, and I think when you've become the leader you'll have diplomatic and political decisions to make such as rebuilding Skyrim's Ruined Cities such as Helgen etc, and depending on which side you chose from the beginning you are to destroy the rest of The Empire's Camps or The Stormcloak's Camps in a few final battles to prepare for the last final one against The Thalmor and The Aldmeri Dominion in their own territory;
An assault on The Summerset Isles taking each and every Elven City from boarding War Ships after taking ether what would become Skyrim's Capital where you rule from ether Solitude or Windhelm's docks. And when you've invaded the Elves you could have the choice to completely secede from the rest of Tamriel making Skyrim an independent kingdom after leaving The Summerset Isles in ruins, Or create your own new Empire by taking Emperor Titus Mede the IInd's throne in The Imperial City ether by force with The Blades or persuasion because you're the Dragonborn, The rightful Emperor; but that is only if you haven't already assassinated him with The Dark Brotherhood ofcourse, If you have, you should be able to just become The Emperor as easy as that by simply going to the Imperial City for a meeting with The Blades and The Elder Council and they decide, and then when you're The Emperor of Tamriel...
It's your job to defend Three of your territories; Skyrim, The Imperial City and The Summerset Isles from even more dangerous Dragons such as Auriel/Akatosh attacking and wanting to destroy the world which in in the ending they do, and could close the Elder Scrolls Series leaving the World of Nirn destroyed in the Fourth Era, Ether that or you save the world and you carry on your life as the Dragonborn HighKing or Emperor doing small repetitive quests such as taking over Bandit Forts, Elven Ranger Camps etc etc... Well that's my brilliant idea for a Third Expansion anyway and I'd hope that every word of it happens and is released! It would be fluffing mint! but if not and The Third Expansion is something completely different then I'd be excited to see what Bethesda throws at us anyway!