The Empire or the Stormcloaks should easily be able to defeat the Thalmor and even successfully conquer Akavir this time. What you guys think? One Dragon lorewise is already enough to cause problems to a company of troops and even large group of mages. If the Dragonborn could gather even just a dozen, than that would be more than enough to steamroll through thalmor and akavir forces.

I don't think a dozen dragons is enough. The Tsaesci are experienced dragon hunters, and the Aldmeri Dominion is actually much more powerful than Skyrim (the game) would suggest.

Well for one it would never happen because it would make the dragonborn far to important for him or her to be a complete unknown in the next game.

Dragons can also be killed, and have previously be killed by ordinary mortals. I would imagine that they give an edge, but the dominion with probably quite a few powerful battle mages should be able to handle them all the same. Akavir has experience with dragons so they too should be able to handle any threat of dragons. They aren't capable of burning tens of thousands of people at once, and as such they don't offer that much, if any, advantage over a group of experienced and powerful mages.