Bench warmer Danzou Sharingan theory
Supposition: I'm not denying that he may be Tobi, Madara, Izuna, Obito, etc. We don't have enough evidence, but those are good theories, and this latest result supports those, but they don't dismiss the argument below.
I am merely writing this to have an alternate theory that explains the Sharingan. If it turns out he's not one of the above, we need an alternate explanation. Perhaps some could call this the:
"bench warmer Danzou Sharingan theory"
1.) If we recall, according to Madara, Danzou ordered the elimination of the Uchiha along with the elders. Given that, there would be a lot of bodies with Sharingans in them. It's not like they were Ameratsu cremated.
2.) People have mentioned that it was the same medical technique by Rin except performed by Kabuto to implant the. Kabuto is an expert medical ninjutsu and believed to be a leaf shinobi (e.g. Chuunin exams), so that's perfectly plausible. He could've entered the village, performed his jutsu and left unnoticed. At that point maybe the Sharingan worked on/off. Perhaps it can't be turned off now, which is why he needs Kabuto to help heal him. Interestingly, I would think that Tsunade could've helped Kakashi unless the only way to do so is a forbidden jutsu she doesn't know but Kabuto specializes in.
3.)Finally, I believe taking a Sharingan eye rather than being born with one fits Danzou's personality better. He'll do whatever it takes to become Hokage and more (now that he is Hokage), and I think that taking an eyeball is in line with that. Further, his plans seem to backfire (e.g. Nagato, Sasuke, and Orochimaru). This would be consistent with the idea that he can't turn it off like Kakashi.
I'm not going to push this theory in front of the others (Danzou is Madara, Tobi,..., Obito). I'm happy with it as an alternative theory. To me it feels simpler than the others, but I'm curious what you think.