*May contain spoilers if you haven't read the manga*
Theres a couple of reasons why I agree that naruto will someday unlock the rinnegan:
First point is tobi said during his battle against konan that the rinnegan was his and he gave it to nagato when he was pretending to be madara, but during the series they mention a few times that madara battled with the first to get his power, at first I assumed this was his cells/wood jutsu - I think he only considered the rinnegan his, only because he won it as a spoil from the battle. Now I think It was for his rinnegan that he knew the first had, the reason for him wanting it must be because of what was written on the secret rikoudou sennin tablet in the uchiha village, the one that only shows information if you have strong eyes.
More evidence would be that he gave it to an uzumaki clan member rather than giving it to an uchiha, who, if you think of it as an uchiha ability - would be able to develop it better with that logic. He knew that nagato would be able to use it properly, and so he could use him as a pawn if it was needed. all of which have massive amounts of chakra and are able to control the tailed beasts easier ( like kushina who possessed this ability, also naruto's mother).
If you go back to the tale of the two sons of the sage of the 6 paths that tobi tells, it makes sense that 1 received the rinnegan and ruled with the power of love or whatever, and the other brother got the sharingan and ruled with power. uchiha are the descendants of 1 brother we know that much, but its made pretty obvious that the uzumaki clan(closely tied to the senju, or originally part of it) are the descendants of the other brother with love power. it would make sense for them to inherit the rinnegan. both the uchiha and the senju were bitter rivals before the age of the first and madara forming konoha, you could prob assume that this had continued since the death of the sage of the 6 paths, and the rivalry stemmed from the fight over who would have the right to rule.
they made it pretty clear in the manga that both the power of the uchiha and the senju are needed for the eye of the moon plan, and you even see tobi with both the sharingan and the rinnegan... if the rinnegan was the evolved version of the mangekyou sharingan, then wouldn't he be using both of the rinnegan that he got from nagato? instead he uses 1 of each...I think this makes it pretty clear this ability belongs to the senju/uzumaki clan. the moon is supposedly the body of the monster, which the sennin sealed as the moon. His power was split between the brothers./his sons. if you go by this the rinnegan is a power of the other brother and not the uchiha brother.
more signs is the sign on the back of konoha ninja is a spiral, like the eye - the symbol that is the symbol of the whirlpool village, which the uzumaki clan used to rule. the clan that nagato is also from. Konoha also wear it as a sign of friendship to the whirlpool village, even though it's been destroyed. it'd make sense if the symbol of the village was the symbol of the power of the original leader.
even naruto's new form with the 9 tales chakra looks like the rikoudou sennin in the pictures earlier on. with the yin yang symbols around his neck and the cloak. killer bee doesn't have this ability I think because he is not a direct descendant of the sage, while naruto is.
They say a few times i think that the rinnegan is the final form of the sharingan, but I take everything tobi says with a pinch of salt, since he's basically lied about his identity all the way through the manga, he's prob twisted the truth about the eyes as well.
also - the sharingan allows the user to use very high genjutsu, amaratsu/fire techniques, susana'o(or however you spell it), and space/time ninjutsu . Nagato possessed none of these abilities. all were Life/death, gravitational, summoning, the ability to use all 5 elements, and the ability to link the eyes of his "clones" - all of which use large amounts of chakra to utlitise, which ties in the the younger brother obtaining the "body" of the sennin. It doesn't make any sense that you would possess eyes at a higher level than a mangekyou sharingan and not use genjutsu, you would gain power, not lose it. a lot of the senju/uzumaki are skilled with clones and summoning techniques. (thinking naruto, kakashi, jiraiya, tsunade, the third - most of these can also use the shadow clone technique apart from tsunade, naruto with the strongest ability with these since he is directly related to an uzumaki clan member- a clan member who was particularly special as she was selected as a jinchuuriki.).
ANOTHER thing, if you look at the senju symbol it even looks a bit like a set of rinnegan, mix it with the uzumaki symbol and it'd look just like it. http://naruto.wikia.com/wiki/Senju_clan
either way the manga is amazing at the moment and dying to find out what's really up!