A couple of the recent threads made me think about the Senju DNA and Senjutsu, what if when Madara added Senju DNA it was just an inefficient way of getting what he wanted. Perhaps the Senju DNA gathers Senjutsu automatically in a passive sense? It would explain Obito's absurd durabilty, much like Naruto's sage mode.
If Madara had become a Sage ie using natural energies he could have awakened the Rinnegan without splicing DNA and all that jaz. Is it possible that the method of unlocking the Rinnegan has jack all to do with Senju DNA but all to do with Senjutsu Chakra just like the Juubi. Madara himself did not seem to know how he unlocked the Rinnegan. Perhaps it just took that long for the senjutsu to build up to an acceptable level for it to unlock. I don't know, but if it was just senju DNA why did it take so long for him to unlock it?
For example the Elder son was said to inherit the "eyes of the sage"*, what is that but the Rinnegan! Why would it be impossible for the elder son and his descendants to unlock the Rinnegan without Senju DNA it makes no sense! If they inherited the Rinnegan they did!
*Lol another play on words, "Eyes of the Sage"?? This is just so stupidly obvious....you have to be a sage (and a descendent of the elder son) to unlock the Rinnegan...its got jack to do with Senju DNA...
Btw I'm referencing the Juugo Sennin Mode thread thats what gave me the idea.