But why can't Danzou just be Danzou? Doing his thing.
Cuz Danzou is old, uses a cane when he walks, had his arm injured fighting for title of hokage and ost his right eye. Hence a transplantion was needed and which eye is more suitable then the sharingan?
But Tobi seems to be younger, and has the same hair style as Obito had (when if) he was crushed. Couldn't it be that Zetsu came to devour his body but found Obito alive but severly injured. In someway healed him and that's why Tobi has the mask displaying only his right eye which was not transplanted. Tobi / Obito should be at the same age as Kakashi and from what I've read and seen that is most probable from the movement of Tobi.
It might be a plot twister this. Or perhaps it's just as easy as Obito is dead, Danzou is danzou and Tobi is Madara. Perhaps Tobi / Madara only use the mask as a disguise and therefore have two sharigan eyes / EMS.
Well it's interesting to see what lays within the future of Naruto, exciting indeed. I remember the discussion of Minato being Pain a while back, that wasn't the case, clearly. So lookin forward to getting some answers!!!