I've got a really interesting idea on the main quest. The history books of Tamriel speak of this happening from time to time, so it would be really interesting to actually explore it as a game player.
I call it "The Elder Scrolls VI: Akavir," and no, the game doesn't take place in Akavir (though, to be honest, I think that would be an interesting side game, a la Redguard or Stormhold). Instead, about fifty years after the events of Skyrim, Akavir learns of the Empire's weakening condition (cuz, ya know, it's not like they have spy sattelites or anything; these things take time), and decides that now is the time to strike, so all four of the Akaviri nations (none of whome would be playable, btw, just like the Greybeards or Dremora) invade Tamriel, starting with the westernmost province of Tamriel, the Summerset Isles.
As the sole survivor of the initial Akaviri attack (which serves as the tutorial quest), you tell your plight to the King of the Altmeri Dominion, who is forced to swallow his pride and request assistance from the other Tamrielic nations. You are tasked with going to the Imperial City and getting Titus Mede III's military cooperation, and subsequently going to the other capitals to unite the Tamrielic militaries for a temporary alliance against the Akaviri invasion.
Yes, rather than taking place in Akavir, this game allows you to explore ALL the provinces of Tamriel! Ok, sure, that was done in Arena, but in exchange for being huge, the game world was largely empty. Since then, Elder Scrolls has developed a reputation for being both massive and sprawling with content.
But, storyline wise, I think it would be interesting to have an Akaviri invasion in an actual Elder Scrolls game, instead of just reading about it in lore books. Thoughts?