For Tobi to be Obito, Obito had to have an upgrade in body or mind and there comes Zetsu. Somehow, some parts of Madara, his memories and feelings have been incorporated into Obito for this all to be true. And I find it reasonable for Tobi to be Obito (not fully but in body) because of the clashes he's had so far with Kakashi, and the fact that Kakashi is on that battlefield atm.
However, my three favorite candidates for Tobi are, Obito, Izuna and Kagami. I think I wrote something about Tobi=Izuna, and that it might not be probable but oh yet possible. I haven't however tread into Tobi = Uchiha Kagami yet and I've read some interesting things and I'll try to lay it down for you. Well just look at this picture:
Now, it makes a little more sense being that Tobi is so old... He is as old as Danzou and Sarutobi were, and we all know how strong they were. Wouldn't an Uchiha that was in the same team as them, be extremely strong as well? Furthermore, Kagami was the disciple of Tobirama Senju, the 2nd Hokage. (Not going to mention the "Tobi"-"Tobirama" but you get it). When Minato fought Tobi, he concluded that Tobi used space & time jutsus that were probably even better than his own and Tobirama's (the one who invented that shit). Couldn't then Kagami have also learned it and been a proficient user of s/t jutsus?
During the Tobi+Kyuubi attack on Konoha, in a panel where Hiruzen shouts that they have driven the Kyuubi out of the village, you can see all of the members of Tobirama's team, the two old kage ministers, the Hokage Sarutobi and even the fat Akimichi, but Danzou and Kagami aren't present.
So what would the motive be for Kagami to hate Konoha, and try to make world peace (in his own twisted way). Well, we know that he had experienced war before and Itachi did some crazy shit to not experience it again, Kagami could thus be the same product of the shinobi world as Nagato and partially Itachi were. Furthermore, the coolest idea for this theory is that...Tobirama Senju was Hashirama's younger brother. And we know that he lived during the timeline when the Uchihas fought the Senjus...So let's say that Hashirama and Madara were approx the same age, then Tobirama and Izuna would also be roughly the same age. And we see Tobirama training the next generation of Konoha shinobi, and there we find Kagami. Meaning, that Kagami's generation of Uchiha is the one that followed Madara and Izuna's, implicating that Kagami could well be the son of either of them or a nephew to them. This could answer the question why he acts the way he does, how Kyuubi seems to think that the Tobi chakra is Madara chakra. How Tobi knows how to control Kyuubi, how one is able to get the Rinnegan, as Madara had the Rinnegan right when he died.
. How the **** does he know how he has lived his life? He has seen him how many times - 2 at maximum?
