Since I wasn't very satisfied with Tobi being just Obito, I've thought of this idea.
I think that the Zetsu that is now half of Obito's body is a Madara bunshin injected with Hashirama's DNA.
Maybe Madara somehow created an "immortal" bunshin. Meaning a bunshin that won't disappear when hit. And used it in his research for the Rinnegan. And because it was his bunshin, it also has his thoughts and memories. That's how Tobi knows a lot about Madara and even Hashirama. And when Tobi said that he is just a shell of his former self, he was really referring to Madara and not Obito. And that's how he made Kisame and even Itachi believe that he is Madara.
At some point i always believe that there is a part of madara in tobi , alias obito . I know obito has skills and yes he can advance to certain level just as sasuke advanced before see naruto at the beginning of naruto shipp. Due to sharingan . But i believe not that much as to totally forsaking his life experience with 4th hokage and comrade even though kakashi mistakenly or aomehow killed Rin . Yes , Itachi advanced so early as to kill his parent but he did that with tears . So i think there must be something else controlling obito while being tobi ,
"Intelligence is knowing when to do how " ~~~ James Alabi
I agree with some parts of this. I do believe the current Tobito we are seeing has both sen and uch combined into his one being. I also believe somewhere, somehow the original madara is still alive. Yes I know he was summoned back with edo tensei but we know thats an altered version of madara based on what Kabuto said. However, throughout the series references to madara have never confirmed his death. It's almost like a running gag that he's a shell of his former self, or "not what he used to be" ..Madara even calling himself a death god and questioning if he's even "madara" anymore.
From all this, I think at one time (maybe even before he found obito) madara transcended his own life and became a new entity of course that is immortal but also technically cant be considered madara uchiha anymore.