[Theory] How Naruto will get the Rinnegan
I think Kishimoto is trying to make a contrast between Madara and Hashirama's battle, and Sasuke and Naruto's Battle at the Valley of the End.
You know how Madara and Hashirama had that epic battle, and Madara, the Uchiha, lost to Hashirama,the Senju?
I think the contrast is that Naruto is the good guy who lost, but intead it was the Uchiha, Sasuke, winning. I think somehow Naruto, from that epic battle, unknowingly acquired some Uchiha DNA. I also think Naruto is has Senju-type DNA in him. So I think out of the blue, we will see him awaken the Rinnegan. I also have thought that Naruto in his bijuu mode is equal to the Sage of the Six Paths. Both Sasuke and Naruto will awaken it. Mark my words.