Pretty sure they abolished Slavery following the Red Year in Morrowind, as the Dres that were the biggest reserve of support for slavery were all eaten by the Argonians when they bulked up on Hiist sap so the Hlallu king had enough influence to end the practice without the possibility of a Dres-Redoran-Indoril uprising.

Also, Ulfrick is a huge dork and is not good enough at break-dancing to defeat the Thalmor in the one arena that truly matters. He's also presumably racist towards monkeys, and we're gonna need the Imga if we're going to pull this off. Even with Numidium Tiber Septim could only force a draw with the old Altmer. What chance do we have without a bunch of ballet-doing gorillas?

I really think our best chance is quickly putting down the Stormcloak revolt then using the death of Emperor Mede to steer a charismatic Bosmer leader on to the throne, then use mixed loyalties in Valendwood as a lever to break Elsweyr off from the Dominion. If the Empire can demonstrate that it's a credible challenger to the Dominion we can probably get the Hist on board purely through shared interest in maintaining the status quo of reality, then start constructing Battlespires in the northner parts of the Tamriel in preparation of eventually invading Summerset Isle from the one direction they're not expecting; space Oblivion!

If we can figure out where Vivec fucked off to and assemble a strategic mythic alliance of Paarthunax, Vivec, the Mane, the Dragon Born, the Nerevarine if they're still around, and whatever other quasi-divine pro-existing legends are wandering around I think we have a good chance of yelling "TAMRIEL EXISTS" loud enough to deafen the Aldmeri high wizards to all other possibilities.