~Hagoromo Otsutsuki Theory~ ( Rikudou Sennin )
So, this guy SOLO'd the Juubi.
Well now we know that he was the first person BORN with chakra in him.
from this branches my theory.
Hagoromo ( Rikudou ), Had EVERY Kekki genkai.
being the first and only person to have been born with chakra in them, he is the blood source for all the Kekki genkai ( Unless Kaguya, his mother, had other children )
This obviously excludes the Sharingan/Byakugan ( Just mutations of the Rinnegan from his sons to his sons family )
He was the first person to have the Rinnegan, meaning he had EVERY CHAKRA nature, further backing up my theory, he could easily combine the natures to make new ones, he had the potential and they had to start somewhere.
so unless Kaguya had other children, he was the source of all the Kekki genkai clans.
Also, every ninja in Naruto is distantly related ( Think about it, 1 person had chakra in them and they all do now..)
Huge incest fest.