I'm asking in context of lore.
The Dragonborn is like the third in command, right? It probably means that if Galmar and Ulfric are to perish in an unfortunate accident, s/he will be in control of stormcloaks. However, I wonder if DB can even vie for the kingship? Or would a new high king be chosen among one of the Jarls, and DB would become something like a military supreme general?
What would happen to DB's position if both Ulfric and Galmar died, assuming that s/he is ambitious and we ignore non-civil war questline(including defeating aludu'in and becoming the savior of the world)?
I don't think DB has claim to the throne, but s/he might be able to become a grey eminence with stormcloaks under his control. On the other hand, if s/he sided with the empire, since legionaries loyalty is to the empire, not to general Talius, it will be extremely difficult to personalize the army.
I'm digressing here but that's why I usually go with SC, cuz you have a better chance of gaining more power. SC are loyal to Ulfric, which mean if he dies their loyalty can be directed to another person, which could be DB in this case.