The hyuga clans style is specifically designed to target the humans chakra points, they could be the first people to find out what chakra was and the sage is the one who came out with it and created ninjutsu.
I mean think about it, when the sage came the world was in chaos and war, ninjutsu didn´t exist so what was there? Yes, taijutsu, the ninjutsu you see currently with the hand signs didn´t exist but hand-to-hand combat back then was called ninjutsu and weapons of course but there wasn´t any fancy stuff we see currently in the narutoverse.
Jiraiya did say that so6p created "modern" ninjutsu so it means there was an old ninjutsu.
Now it´s officially confirmed that the sharingan goes to the rinnegan and here is my theory.
The hyuga clan could see chakra and their tajutsu(old ninjutsu) was advanced and one of them humped an uzumaki and gave birth to so6p with sharingan.
The rinnegan didn´t just come out of nowhere, no..the sage started out with the sharingan, developing it and it was only after he got the rinnegan that he came out, stopped the war, became a legend and so on.
It would explain kakashis famous claim that the sharingan come from the byakugan and it would explain how the sage got the rinnegan since he was the first person to ever do it.
So for people who think if nagatos was given to him and madara got it through the sharingan then how the sage got his....here you have the answer.
The Sage was an Uchiha!! The first Uchiha ever!! A true Uchiha!! Both body(Uzumaki) and eyes(Hyuga)! The current uchiha have only the eyes, they are a half-breed, the senju come from a full-blood uchiha think about it!
Why would you need senju blood to use izanagi, why would you need senju blood to unlock an uchiha technique?!
Because uchiha and senju are 2 sides of the same coin! They complete each other!!
The sage could use a more advanced form of izanagi because he is the perfect uchiha! He has the best uchiha eyes(rinnegan) and the best body(uzumaki), he does a more advanced form of izanagi, the creation of all things!!
Btw, this is why the rinnegan is purple, the byakugan is almost the same color and for the sharingan(red) to change into rinnegan(purple) tells you that it has something to do with the hyuga!!
This is my theory!!! So don´t bitch about it!! If you agree, good for you, disagree then **** you.