Quick theory on Tobi
Hi there, this is my first post on the forums and though i'm not certain if this theory has been put forward already i felt i'd share it.
My personal opinion is that Tobi is Madara's younger brother (yeah sorry i can't recall his name) and here's why i think this.
Ok so we know that Madara was "given" his eyes by his younger brother and we can all assume that he felt pretty bad about his brothers later demise, though it was a necessary sacrifice for Madara to continue in his position of dominance over the Uchiha clan. Due to this and the obvious fact that it was his brother we can guess that if he had the chance and ability to revive his brother he would do so right?
Madara also claimed to have unlocked the Rinnegan shortly before his death - we'll come back to this in a second, just keep it in mind.
There's a lot of speculation between the relationship of Madara and Nagato, however i think we can agree that at some point they encountered one another and i'm going to go with the fact that Tobi (madara's brother) gave him his Rinnegan after having unlocked it, this is where we go back to Madara's Rinnegan.
Following his attack on Konoha and after speaking with Naruto, Nagato used his Rinnegan to revive those that had died during the battle, this wasn't an Edo Tensei style resurrection it was pure life restoration putting them back to how they were before in every sense of the word.
I'm going to speculate that once Madara unlocked his Rinnegan he felt he owed it to his brother to revive him, maybe he knew and maybe he didnt but i think during the revival or shortly after Madara died, maybe this is the cost of the technique on its user.
Having been revived his brother (Tobi) now with the knowledge of the affects of the Jutsu needed a candidate to perform the technique on his behalf, and transferred his Rinnegan to Nagato knowing he could bend him to his will and get him to revive Madara at a later stage.
I can't speculate on the reasons why it didnt happen sooner, but anyway this is my theory on things hope people find it at very least and interesting speculation.
-Adam