Stormcloak racists and xenophobes on one side and religious persecution under the Imperials. I think he'd use both sides to his advantage and birth an empire of his own. That's really what he did in the first place among rival factions.
That's what I think. I kind of wish you could use the title of Dragon-born in Skyrim to trick others into thinking you're Tiber Septim reborn, stand up during the truce conference, say "I'm tired of your [censored]," and proceed to whip both General Tullius and Ulfric up and down the room with the tip of your shoe.
They'd both have a reason to be loyal to Tiber Septim, so.