The others theories I have suggest that Madara is not in fact Madara Uchiha. One could be that Tobi/Madara is actually Obito Uchiha (as some people still believe) and Kabuto had summoned the real Madara Uchiha which would reveal him as a phony to the entire world which would foil his plans. Not to mention that if Tobi is indeed Obito, the real Madara Uchiha would be powerful enough to defeat him. Another theory also suggests that his identity is Obito but who Kabuto summons is not somebody who can stop him using power, but somebody who can cause him emotional anguish. I'm talking about Rin, the girl that Obito was in love with when he was teammates with Kakashi. It could also be an entirely new character that we've never seen but could foil Tobi's plans. Another theory is that Tobi is not Madara but actually Madara's younger brother Izuna pretending to be Madara and the body in the coffin is the real Madara Uchiha. Tobi might also be a character that we've never seen that is simply pretending to be Madara and the body in the coffin was Madara himself which would question the plausibility of Tobi's claims of being Madara. What really makes me question Madara's identity was that Kabuto told him not to worry and that he won't tell a soul which strongly implies that Tobi has some sort of secret. The most unlikely theory I believe could be that Kabuto had summoned the Sage of the Six Paths himself. But that would just end the entire series, not to mention that the Sage's body should have already rotted away to nothingness.
So what are your theories on who Kabuto summoned? Why do you think he summoned who you say he summoned?