Want to know who the supposed Madara Uchiha truely is?
First of all, in manga chapter 502, Minato confirms that the man in the mask is not Madara. He ask him if he is Madara Uchiha but then says he can't be because Madara is dead. To which the man in the mask replys, "well... Who knows?..." Minato then says that it's not important who he is but instead asks him why he is attacking Konoha. The man then replys that it was both a "whim... and a plan both for war and for peace".
In manga chapter 241, Kakashi and Obito are arguing about whether or not to save thier team mate Rin. kakashi tells him that shinobi that abandon the mission and break the rules are scum to which Obito replys " Sure in a ninja's world those who violate the rules and fail to follow orders are lower than garbage.... If I'm scum the rules are no good to me and if breaking them makes me the wrong kind of shinobi then I'll crush all the so-called shinobi." This hints at a darker side of Obito and perhaps the beginnings of his master plan to cause destruction to bring peace. Remember, he did grow up during the third great shinobi war.
While fighting, Minato also comments on how the man is similar to Madara but not him. Also, it becomes evidant that the man in the mask knows how Minato fights. He also had a careful and systematic plan in place to get Minato out of the way so he could take Kushina captive in order to extract the nine tailed fox. When Minato reacted the way he did when the man threatned the newborn Naruto, the man comments on how he expected that from the yellow flash, implying he knew a little bit about his personilty.
Minato was Obito's teacher so it would make sense that Obito was familiar with the Fourth's persoanlity and fighting style.
Also the man in the mask has the ability to control space and time with his sharingan. He has displayed sevreal times this technique when he sucked his enemies into a swirling vortex created by his sharingan. Kakashi's Kamui is similar. When chasing down Deidara, he sucked Deidara's arm into a swriling vortex and ripped it off, sending it into some other demension.
Kakashi got his Sharingan from Obito so it makes sense that if the man behind the mask was truly obito then he and Kakashi would have similar Sharigan abilities.
The man behind the mask only reveals his left eye. Kakashi recieved Obito's right eye after Obito was crushed beneath a bolder. Supposedly Obito's entire left side of his body was crushed. The man has stated before he needs to make himself more complete and in recent mangas he was able to detach his left arm easily. This implies that perhaps his left arm was a fake and by making himself more complete he means restoring the parts of his body that was crushed. Also no part of the man's body is exposed. Underneath his robe he wears a full suit that covers his neck and head and his arms. Why is he fully covered? What is underneath that suit really?
My next statment I have no evidance for, but perhaps the suit is like a stand in for his body or at least the part that was crushed until he can somehow make himself complete.
Also in maga 509, when Konan and the man verse, she blows up part of his mask revealing more of his face. His eye shape and hair are similar to that of Obito not the original Madara. Also it reveals that the man behind the mask is young. If he were the real Madara he'd be old and wrinkled.
Obito may have been similar to Naruto but perhaps these two who were similar in the beggining and who want the same thing for the world, which is peace, represent the two opposite extremes for the same cause.
That is my theory and most of my reasoning behind it...