Do you consider Dovahkiin powerful enough to help Empire/Stormcloaks defeat Thalmor?
After completely annihilating an entire Fort filled with Thalmor, I don't see much to stop the Dovahkiin, should he be given the chance.
If he sided with the Stormcloaks, they won the war. And the Empire has to be wondering, "If the Dragonborn is against us, what does that say about us?" And the Thalmor know exactly who destroyed their embassy and fortress, and that he did it alone. In addition, they now know that no matter how many assassins they send after him, they will fail to kill him. Two capitols are abuzz about this time with worried "what now?" discussions. So, if nothing else, plans for domination will have to be reexamined in light of recent startling developments, or put on hold indefinitely while this "new problem" is sorted out.
So, out of the blue, Dovahkiin has already altered the course of two "empires", without monumental effort. He should also serve as a rallying boost for the Nords, being a walking, breathing legend from their own mythology, and on their side. You know there will be Imperial factions aching to make him the Emperor, as well as those devoted to destroying him. So, all in all, if Beth follows through (not holding my breath), we could see epic struggles ahead; politics, war, economic upheaval, social disruptions, a complete rethinking and perhaps retooling of "reality."
Won't happen, though, we'll just get more gee-whiz graphics and better looking water, maybe some new skill sets or perks, and we'll be content to keep arguing and discussing "what if" scenarios that Beth should have taken into account.