Theory about the Ten Tails, dojutsu, and the So6Ps
Clearly the Ten-Tails has some sort of Rinnegan/Sharingan hybrid for an eye.
It is known that the Sage of Six Paths became the jinchuriki for the Ten-Tails.
It is known that somewhere after this, the Sage of Six Paths knocked some woman up that gave birth to two children that were the descendants of the Uchiha and Senju clan.
It's known that the Sage of Six Paths, towards the end of his life, split the Ten-Tails into 9 separate tailed beasts and finally sealed the body into the moon with Chibaku Tensei.
It is known that the Uchiha Clan possesses what is called the "Curse of Hatred". This usually causes them to do some pretty bad stuff.
It is known that the tailed beasts can be controlled by those that have a powerful sharingan.
My theory is this:
The dojutsus all come from the Ten-Tails. I believe that the Sage of Six Paths did not initially have the Rinnegan, but instead, he acquired it when became jinchuriki of the Ten-Tails. Some time after that, he had children. I believe the child who is the Uchiha clan ancestor inherited part of the Ten-Tails which was inside the Sage of the Six Paths. I also believe that they inherited the "Curse of Hatred" because of the chakra they inherited from the Ten-Tails. To further prove this point, I believe the reason the Sharingan can control tailed beasts is because the nine tailed beasts are nothing but parts of the Ten-Tails. And since I believe the Sharingan comes from the Ten-Tails as well, it is simply a way of the Ten-Tails' chakra controlling each other and possibly trying to become whole again.