My theory is that Danzou and Madara had a fight at some point in the past and Danzou was able to steal one of Madara's eyes, hence the reason for Madara's mask, as his face, I assume, will be revealed as horribly disfigured as well. It would also explain Danzou's screwed left side as Madara wouldn't go down without a fight.

Just throwing it out there =P