Tobi is Izuna.
Tobi is always behind the mask and the reason why he wears that is simple. He doesn't want to show his true identity. Also, Zetsu always calls him Tobi and Tobi was the one who created Zetsu, so Zetsu might know his true identity or at least that he's not Madara.
Then the reason why he's Izuna:
Hashirama indeed killed Madara, but because they both were so damn powerful, Hashirama took permanent damage. That's why he gave the tittle of Hokage to his brother, Tobirama, because Hashirama was at that moment weaker than his brother. Izuna didn't die. I think that Izuna and Madara had a close relationship and Izuna gave his eyes to Madara. After that Madara gave him someone else's eyes and doesn't it sound bit weird that Izuna died, because he fought without eyes? It shouldn't be so hard to take some random eyes and give them to Izuna. Anyways, there are two possibilities how Hashirama died.
1) He took so much damage against Madara that he couldn't survive.
2) Izuna killed him. Madara was so important to Izuna that Izuna couldn't forgive Hashirama.
The reason why Izuna helped Itachi to get rif off the Uchiha clan is simple. The Uchiha clan betrayed Madara and admired Hashirama more than him. Izuna couldn't forgive Uchiha clan either and that is the reason why he helped Itachi. And I think Madara really wanted to protect the Uchiha clan, even though they betrayed him.
When his vengeance was over he had nothing left. Look at him now, he's trying to control every human being and take over the world.
All what he has done is because of the love towards his brother, Madara. He goes by his brother's name, because he wants that everyone thinks that Uchiha Madara is the greatest ninja.
Also this is quite similar to Itachi's and Sasuke's case. Itachi wants to protect Konoha, but Sasuke can't forgive them after everything they have done to Itachi.
And the guy in the sixth coffin is Madara Uchiha. Young Madara with EMS would scare anyone. Also Tobi asked ''How did you do this'' and Kabuto responded ''Well you know''. To me it sounds like Izuna hid Madara's body , but Kabuto found it.
Hopefully you liked my theory and think that it's plausible
