I get why people say the Dragonborn should rule skyrim but...ut makes no sense. Sure he's a hero who saved the world, doesn't mean you get to rule a country. All the dragonborn knows how to do is fight and kills dragons, I doubt he/she has any idea or experience on what it takes to lead a country, both politically, economically and militarily.
Also keep in mind the dragonborn would have to be nord as well as have some claim to the throne (ie. Be a jarl)
This would be the dargonborn after getting the seat of high king