OMG! That would explain why Madara only has one eye too!!!! Danzo has the other one and somehow Madara was able to transfer his body into Shisui's after Danzo stole his eye. Maybe Itachi knew this...
Okay this is my theory:
Itachi went to Madara and told him that he had a proposition. He needed help destroying the Uchiha clan, and in return, he could help Madara...at that time, Madara was near death because of his fight with the Hokage, and so he said if Itachi could give him a worthy host, he'd help him. Itachi killed Shisui, gave one of his eyes to Danzo, and took away the body. Instead of disposing it, Madara took over Shisui's body, but he only had one eye because of it. Still, he isn't able to fully control the body because it is not his own and there is only one eye. Madara probably assumes that if he transplants the Rinnegan his body will be complete and he will have a combination of the two most powerful eye bloodline limits in the world...EMS and Rinnegan combined, making a power so strong that he'd be able to control the juubi!!!!! This also explains why Madara wanted to take Danzo's eye when Sasuke killed him...he probably wanted both eyes to posses the ultimate eye jutsu. Just a theory, but I think it is plausible.
For those who were asking for pics of Madara and Shisui's face, I found what are as close as it comes to their full faces.
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that is Shisui's face, and remember that if Madara did take over his body, his face would look a lot older than it did when he was young. He would be at least 5 years older than Itachi when he died, and Itachi was beginning to get wrinkles.
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This is part of Madara's face, which doesn't look exeptionally old...it isn't as though he looks like he's 100 and ready to croak or anything.