Though, if the Rinnegan was a by-product having the 10 tails sealed within him, how is it that the dojutsu was able to passed down after to the elder son (and his descendants)? But backing it up, how is it that the venerable Sage was able to retain the Rinnegan if the 10 tails was already sealed, and split into 9 parts? If the Rinnegan is a characteristic of being a host to the 10 tails, how is it that he was able to retain the Rinnegan? Some details regarding that are either lost or, not explained fully in depth
I acknowledge that, however were talking about the combined might of the 9 bijuu here, so much power was granted to RS that he was able to CT a moon, clearly the Juubi was no ordinary Bijuu.
Also, the Kyubi as well as Shukaku had physical affects on both Naruto and Gaara. Gaara was unable to have regular sleep patterns and Naruto has the wiskers which signifies the animalistic aspects of his Bijuu.
Juubi being the original seed and the combined might of all 9 makes me think that granting the user who became comparable to a god special physical traits such as magical eyes wouldn't be that implausible.