Now before I go into details, I would like first to state how you can get rinnegan. This is essential so we can connect the dots.
According to the manga, the equation of the rinnegan is: rinnegan= senju + uchiha dna.
Therefore hagoromo= senju +dna.
However, if you really think about it, Hagoromo was the child of his mother (obvious I know) and that mother was a powerhouse of chakra since she ate the fruit of Shinjuu which had all its charka. However, she herself didn't have the rinnegan. Therefore, how you explain the fact that he had it? Simply, he didn't get it from birth, the only thing he got was monstrous chakra just like his mother. For that reason hagoromo must have got it by sealing the Juubi inside of it. It's preety logical considering the fact that Juubi's eye, which btw has visual powers, is its main trait, and just like every beast, that way Juubi as well, granted his jinchuuriki its powers.
Now you'll go by countering with the equation rinnegan= senju + uchiha dna. However, what are senju and uchiha in the fist place? Since, hagoromo was a person with poweful chakra, how he made his children to have so diffrent and distinctive traits? The truth is that it's not possible. Those traits came from the Juubi itself and it's obvious, I don't think that I need to prove the similarities between of them. So you could say that in fact Juubi= senju and uchiha.
Based on the previous ones we come the conclusion that Hagoromo didn't have the rinnegan by birth but in fact he acquired it after becoming the Juubi's jinchuuriki. Therefore, we now have 2 equations as to how you get the rinnegan.
1. By being Juubi's jinchuuriki.
2. By having both senju and uchiha dna.
Now, let's examine Obito's case, whom you want so much to take as an example to counter my theory. To do that, we need to examine madara's case as well, who is the only one we know that managed to awake it and we even have information as to how he did it.
Madara managed to evolve his EMS to rinnegan thanks to senju dna and his own. Whether or not you need strictly to follow all the steps to get it is something we can't really know, I mean whether or not you can immediately go from MS to rinnegan. Though, you could say that most probably only those with EMS can since rinnegan is an eternal dojutsu, just like the EMS and therefore EMS' "eternity" could be a clue that you need indeed to have it in order to evolve your eyes further.
If the above is correct, then Obito simply couldn't awake it due to the lack of achieving EMS.
As for the first, alternative way I proposed. You say why he didn't get it by becoming Juubi's Jinchuuriki. Do you remember Madara's statement? he awoke it during his late days. From where has it derived the conclusion that those who become Juubi's jinchuuriki, they get it right away? As far as I remember the only thing we know about Hagoromo is that he had rinnegan but we don't know neither the when nor the how. However, we do know one thing: All the rinnegan abilities that he used, he used them during his very late days. It's then that he divided the Juubi, it's then that he sealed Gedo Mazo by creating the moon. (And it's then when he created his sons the same way he created the nine tailed beast? I can't back up the latter based on strong evidence so let's leave it as it is). Point is that at this moment I see a strong similarity between madara and hagoromo; both of them started to use it when they were about to die.
The reason for that is that they couldn't before, exactly because they haven't awakened rinnegan yet. The fact that Hagoromo decided to divide Juubi during his latest days could very well fit to the image of trying to do the right before leaving this world but couldn't also mean that he chose to do it at that exact time because like Madara, he didn't have access before?
Therefore, Obito didn't awaken it because he didn't reach his old age. Ofc, no one has said that you need to reach the latter in order to do it. It can also very well be interpretted that you need to be at the point of being about to die - Obito wasn't. Even when alll the bijuu were extracted, Kurama mentioned that Gedo mazo's lifeforce was strong enough to keep him alive (and he was planning to revive everyone at that moment!). Now that the gedo left, he can still survive because of Black Zetsu (for the time being).
Now to come to the main point of this thread and how naruto can awake rinnegan
I have already proven that by being Juubi's jinchuuriki and by being about to die, you can unlock the rinnegan. Naruto has already experienced the latter (even though Kushina experienced the same thing with Naruto, she did survive for some time only to be killed by Kurama. We don't know though for sure as to whether she could be cured or not).
So the only thing is needed now is to become Juubi's jinchuuriki. Ofc, he can't become of the whole thing but I think you know where I'm getting with this. Naruto had already got chakra from 7 out of the 9 tailed beasts and now Ichibi and Hachibi (or at least one of its tails, lol) are there to provide the rest of the ingredients. Minato can be used as a substitute for the loss of half of Kurama by providing his own.
For those of you who will say that Gezo sucked all the chakra naruto had got from the other beasts, well learn that even if that happened, it's not a big deal. At a previous chapter Sakura had commented about how when Naruto ran out of charka and the cloaks he had provided to the alliance ran as well, she thought that he was out of juice, however, once he got it back, the cloaks were reactivated as well meaning that there is always something that's left behind. That something can still be enough but in case it's not, the bijuu can still give him chakra again via telepathy, the same way he gave to the alliance once he had more chakra and had come in contact with them in a previous moment.
So by getting chakra from all the tailed beasts he can make a mix of them and therefore enter a pseudo jinchuuriki state of Juubi since the mix would have chakra from all the nine of them. By doing that and by having already being about to die as we saw in the last chapter, he will have completed both of the 2 requirements for one of the ways we established to unlock rinnegan.
I know that I spent too much time analyzing other things in the beginning but if I hadn't I wouldn't be able to establish my theory correctly.
Hope you liked it=D
