He'd smack both sides around until they served under him.

That was on the lines of what I was going to originally say, so I'll try to be a bit less facetious

There's no way he'd support the current Empire (it's not even his anymore) without some serious reform. I think he'd be more inclined to helping the Stormcloaks, create a new Nord Empire and all that, but he might have to set them straight about a thing or two as well.