Board Thread:Theories and Speculation/@comment-27924662-20161219053341/@comment-4912646-20170112123008

Even though the tablet, or at least the section Madara quoted, doesn't mention the Ten-Tails as a requirement for Rinne Sharingan, we have to keep in mind that Madara and Obito's entire plan revolved around getting the Ten-Tails so it's pretty clearly necessary. If you just needed the Rinnegan and close proximity to the moon, old man Madara could have just elevated himself with Wood Release or learned Ōnoki's flight technique and saved himself a lot of trouble. We also know that you need the "power of Rinne", which seemingly refers to a set of two Rinnegan. So essentially you need the power of Hagoromo's body (Ten-Tails/Six Paths Senjutsu) and Hagoromo's eyes (Rinnegan) in order to obtain his mother's power (Rinne Sharingan). Though whether Sasuke's unique single eye Rinnegan and Naruto's non-Ten-Tails-related Six Paths Sage Mode together would work is an interesting question.

We also don't really know if Madara obtained his own distinct Rinne Sharingan or just manifested Kaguya's on his body, seeing as it's the only part of him that Black Zetsu didn't cover when he transformed into her. And there's nothing explaining why hagoromo doesn't have a third eye, unless we assume he just never flew high enough. An interesting theory I've seen around is that the marking on Hagoromo's forehead may be some sort of seal to stop the Rinne Sharingan from appearing, though the anime obviously didn't go with this (they just made it appear kind of randomly without explanation).