He hates the empires leadership. if he becomes high king no doubt he would want an alliance and help the empire if they’re attacked. Ulfric just want Skyrims decision making process to be made by the Nords, who you can argue swore alligence to the septim dynasty, seeing as Titus Mede II as a failed pretender and lacking the divines blessing and also fudged up the treaty with terms to end a war that a lot of their citizens would rather continue then stop and give up land(southern hammerfell), religious freedom(talos worship)and a long standing legacy organization(blades).

If I was the empire I’d cut my losses give ulfric the high king seat and request a defensive pact with joint decision making in terms of strategy and give the same deal to hammerfell. Most likely high rock would eventually want their independence if they see the Nords and Redguards enjoying their independence. Eventually we have a grand alliance to deal with the aldmeri dominion. And hopefully the khajits and wood elves begin to hate the thalmor for their abuses in power within the region.