I think rinnegan = indra chakra + Ashura chakra + EMS eyes + near death experience
It seems Naruto had some of Indra's chakra yet he didn't unlock rinnegan despite having Ashura's chakra and having a near death experience. This is why I think you need EMS to get rinnegan.
Hagoromo is able to appear if someone has Indra + Ashura + kyubi chakra according to the manga, this is said when the sage appears to the kages from Madara's cut off body (shown on Naruto 686 page2).
In the middle right panel in the this pic the Sage says how he was able to appear.
Also Sasuke should be able to gain a 2nd rinnegan from another near death experience if he got Hashirama cells from Kabuto which should mean he has Ashura chakra.
I prefer how Sasuke looks with 1 rinnegan / 1 sharingan so I don't mind if he never unlocks a 2nd rinnegan.
It seems Naruto had some of Indra's chakra yet he didn't unlock rinnegan despite having Ashura's chakra and having a near death experience. This is why I think you need EMS to get rinnegan.
Hagoromo is able to appear if someone has Indra + Ashura + kyubi chakra according to the manga, this is said when the sage appears to the kages from Madara's cut off body (shown on Naruto 686 page2).
You must be registered for see images
Also Sasuke should be able to gain a 2nd rinnegan from another near death experience if he got Hashirama cells from Kabuto which should mean he has Ashura chakra.
I prefer how Sasuke looks with 1 rinnegan / 1 sharingan so I don't mind if he never unlocks a 2nd rinnegan.