1. Favored by the Nine Divines
2. Related to Tiber Septim
3. ??? (is there a another way?)
Now any race can be "Dragonborn" but it is rare.
I couldn't help but maybe think that the Dovahkiin is infact related to the Septims via
"...one of these was the War of the Red Diamond wherein Skyrim sided with Queen Potema against the Empire. The civil war tore the Empire apart and it would take seventeen long years before Potema was finally defeated. As a result of this war there is a strong underground movement called the Horme that believes Potema and her deposed son of Uriel III to be the last of Tiber Septim's true blood and under that principle lead raids against Imperial interests in the province..."
Now of course it is questionable (very much so) because we are not entirely sure that they are dead, I could find nothing saying so at least. Maybe Uriel found that "Lusty Argonian Maid" or some Nord wench etc. or one of his ancestors did, and since the offspring take the appearance of their mother this could explain why your character is "X" race.
Your thoughts and opinions?