Man behind the Mask.
So with coming of chapter 599 whole naruto fanbase world is shocked and rocked to it's foundation. "Tobi is..." threads are now back to bussiness and everyone is trying to figure out who he is. Well, im not an exception so hear me out.
To me, there are only two options now :
1)Tobi is Madara. Muu's body split technique and the whole Zetsu Clone thing could point out to the fact that Madara at some point created backup plan in case he dies. First thing he mentions after being ressurected is Nagato which means he was planning to get ressurected by Nagato.
But as we can see Madara is brought back in the same condition that he was when he died to Hashirama. His battle clothes and age points out to this fact. Also, ressurected Madara kind of looks like an old timer. It looks like he doesn't have a clue about Oro or Kabuto and if he would live more than 100 years to meet Nagato by himself he would definetly have known Oro and would make a comment about him because 2nd Hokage is dead and the only one to ressurect him would be Orochimaru.
In some sense he was insulted by the fact that someone would use Edo Tensei on him, the way he described this jutsu just tells that he is not suspecting 2nd Hokage to do it but yet again he doesn't mention Oro or Kabuto.
This just means that the clone he left behind himself was supposed to get done this whole thing with Nagato.
There's also a chance Madara somehow predicted that there will be a boy with rinnegan and that his alter ego would use him in future to ressurect himself.
There are alot of loop wholes in this theory so that's why im kind of leaning towards second option.
2)Tobi is the real Nagato. He is Madaras son or his secret apprentice. Madara told him a way how to obtain rinnegan which he later would use to ressurect Madara in case he dies. Madara says "it appears that ittle brat has grown". Now really, would you call some random kid who you don't care for a "little brat"? No, you would call that way someone you knew pretty well, perhaps an apprentice or your own son.
What makes me think is why didn't Tobi used rinnegan from the start? Why give it to some Uzumaki test tube called Nagato? Was he was afraid of using Rinne Tensei? Perhaps this technique would take his life away so he needed some test subject, someone who could handle Rinnegans powers and Uzumakis are known for having vast chakra pools and for being incredibly gutsy. He planned on using Nagato to ressurect Madara and then take away Rinnegan from him but that failed and at this point Tobi had a change of heart and instead of using Rinnegans powers to ressurect his father/master he decided to use those powers to become SO6P successor and "savior" of the world. He went "Kabuto" on Madara. Maybe that's why he sh1t his pants when he saw Madara in the coffin?
If Tobi was really Madaras split soul or some kind of clone or even Izuna Uchiha wouldn't his priority be to release Edo Tensei Madara from Kabuto? Instead he doesn't give a shit about that. He just goes with his plans and hopes Kabuto won't double cross him.
Also, there's a chance that the apprentice or son I mention is Kagame Uchiha in disguise.