I dunno if this is the place to put it,but, the manga got me thinking?
Tobi used Izanagi (I think its spelled like that) against Konan, if I can remember correctly he said that only a Uchicha + Senju can use that ability.
And Tobi has some clone-like thing having the 1st Hokage's cells.
I'm swinging for the fences here, but could it be possible Madara was experimenting with
the 1st's cells to achieve Rinnegan, by combining the Senju cells with his Uchicha cell and
and making the two bloodlines "complete". It could be the secret the Uchicha stone was hiding - unlocking the rinnegan.
I dunno if that makes sense, but could explain Oro and Kabuto's interest in Sasuke and
Danzo's experimentation?
Its my first thread, so please bear with anything I might have missed, spelling mistakes, etc. or if someone has already covered this?