Edit: Please note that whenever I say Senju or Uzumaki it refers to both, meaning Younger Brother Descendants.
Rinnegan is not actually an Uchiha trait. Yes, it seems that EMS is a must, or is it, in order to unlock Rinnegan, but it's only half of the way. Uchiha's need Senju DNA too. From Madara and Kabuto's talk it seems that Madara fought Hashirama in order to get his power (DNA) and thanks to that was later on, near his death, able to progress his EMS towards Rinnegan. This makes Rinnegan as much an Uchiha trait as it is a Senju trait. And Since Nagato, an Uzumaki, was able to use it then it means that any Senju/Uzumaki could use it too. Notice that I said use and not unlock.
I have a theory though, that what Tobi actually implanted on Nagato was a pair of sharingans, and since Nagato is also a descendant from the younger brother, much like when Sasuke first awaken the sharingan, in a traumatic situation Nagato awoke the Rinnegan. Why is this?
Well we all know that the Elder Son of the Sage wrote down the secrets of the Sharingan and the Rinnegan in the Uchiha's Stone. It's also stated that these secrets are not visible to all, in fact each time you progress in Sharingan you unlock some more info on the rock. My theory is that Madara only learned how to unlock Rinnegan with the info he got from the stone with the EMS, hence why he was so surprised how Orochimaru and Kabuto knew so much about the stone and what took to unlock the Rinnegan since no one else had the eyes to unlock that knowledge, and he kinda like was the only one with EMS.
So with EMS Madara was able to learn from the stone that Sharingan could progress into Rinnegan. Then he goes after Hashirama's DNA, coincidence? I Think not. Uchiha got the eyes and the chakra but they lacked the body to truly incorporate all of what made the Sage of Six Paths. That body are the Younger Brother's descendants. I think that is what Madara learned from the stone with his EMS.
At this point we assume that in order to unlock Rinnegan, if you are an Uchiha, you need both Senju DNA and EMS. Which is true because without EMS you can't find out the info on the Stone. But from a logical point of view you don't actually need the EMS in order to get Rinnegan. Maybe all you need is MS and Senju DNA? Granted Madara had EMS but is only an extended MS, a safer one let's say.
So assuming this is true, then we have Tobi implanting a pair of Sharingans in Nagato. Nagato then later witnesses his parents being slaughtered, causing him great pain. In theory that is what takes to unlock MS, if you have a Sharingan. As it happened in a body that descends from the Younger Brother it automatically turned to a Rinnegan, bypassing all the sharingans.
It makes sense to me because it accounts for every aspect that is known to us. It also explains why while not being Madara Tobi could implant the Rinnegan in Nagato, only that he did so much implanted the grown Doujutso as he planted the seeds set the stage and then waited for it to grow coming later to harvest it. Tobi had access to loads of Sharingans as we've seen so it's possible.
What do you think?
