How about the possibility that Madara is indeed dead, but another Uchiha has chosen to remain unknown and follow in his footsteps? I figure if Madara was the person who summoned the nine-tails, then he probably did it in betrayal to his village. So, he probably founded his own colony elsewhere, and had lots of Uchiha babies there. Then, at some point, he decides to wipe out the remaining Uchihas in Konoha to ensure that the Sharingan survives only in his new village. At which point Itachi gets involved, pretending to wipe out the entire clan, but purposefully leaving his brother alive to revenge the entire clan for the two of them.
Plausible?
Note the obvious absence of an Obito reference here; he's most likely not coming back.