Sasuke may have the potential to unlock Rinnegan already
Hopefully this theory is not a repeat. Sorry if it is. But with the latest chapter that came out today, I though about this.
As stated in Naruto, in order to be able to unlock Rinnegan, you must have both Senju and Uchiha DNA (See latest chapter for references)
Madara’s Rinnegan awakened
Madara battled the First Hokage and obtained his DNA (Blood). Whatever Madara did with this blood sample, it gave him the potential to unlock the Rinnegan. But it didn’t go according to plan for Madara. His Rinnegan didn’t manifest until his “life span was almost over”.
So, what did Madara do with Harishima’s DNA to combine it to his?
According to the latest chapter, he applied it to his wound. Blood to Blood contact seems to be how he transplanted the DNA of the first Hokage.
Nagato’s rinnegan awakened
Nagato’s has been proven to be of the Senju lineage, so he has one half the formula needed for Rinnegan. Madara choose Nagato for this reason and transplanted his eyes into him when he was young. One of 2 things can be assumed here:
1. Madara’s eyes were sufficient to give Nagato the Uchiha DNA needed to awaken Rinnegan.
2. Or Madara transferred some of his own blood into Nagato at the same time he transplanted his eyes.
Whichever is the case, Nagato obtained the potential to unlock the Rinnegan. Nagato’s rinnegan did not awaken until the day he and his friends were attacked by an enemy ninja. Nagato states that he blacked out during the attack. Perhaps his black out came from a near death blow.
Sasuke’s potential to awaken Rinnegan
Sasuke is obviously Uchiha. It is my belief; he already has Senju blood in him, and has had it for some time.
It has been strongly guessed and maybe even already stated in the manga (I’m not entirely sure), that Karin is a Uzumake/ Senju descendant. The fact that her blood can renew the strength and chakra of others gives credence to this, given the healing properties of the First and Fifth Hogake’s and the fact they are both Senju.
If she is indeed a senju descendant, Sasuke has drunk enough of Karin’s blood to give him the Senju DNA required to unlock Rinnegan. I'm not saying he will, but I think he can already.
The question that remains is this, will Kishi allow Sasuke to unlock Rinnegan and how will it happen? I think Sasuke has to be near death to unlock it.