Ok..to start off..with this image
We see here that danzo has the sharingan..
now i have a theory of which how danzo got that sharingan..
that sharingan i think is implanted..thats why danzo's covering it with bandages because it's always activated..so the danzo obito theory can never happen..obito is a from the uchiha and carries uchiha blood..at any time he can just deactivate it..but what we see is danzo always keep it covered.
but who? whom did he get the sharingan?
easy..from uchiha shisui..remember that uchiha shisui's body has been recovered from what we know as suicide..and that body has been recovered by the uchiha police force..but did you know that the uchiha police force has connections from the anbu?..
now as we know that anbu has connections with the police force..and there is someone who is in charge with the anbu factions..and that is Danzo!!
i believe that danzo got shisuis body and transplanted a sharingan eye to his body.
why did danzo not transplanted both of shisui's eyes?
easy..its a transplanted eye from a uchiha blood from a non uchiha body..if you transplanted both then danzo's chakra is dead..so the solution is covering both of the eyes??..:p guess not
this is where a point of tobi's identity takes place.
i really guess..i really really guess that tobi is shisui uchiha..not madara..i dont know but we know that uchiha shisui is a very talented uchiha and is know as the shisui of the mirage..other is shisui of the body flicker..other is shisui the teleporter..just like what tobi does when he just goes from place to place..and if you observe..all of the uchiha identites in the naruto world..we saw all of their faces..but only one we have not seen yet..shisui..
i think thats why tobi only has 1 sharingan because the other one is with danzo.
but how did that happen
there are 2 missing people who did not arrived at the meeting..those are uchiha shisui and uchiha itachi..i think both of them did not arrive because of something..i think they knew all about the uchiha secrets and knew how to get the mangekyou sharingan..maybe shisui is the greedy one and tried to kill itachi but itachi defended and maybe killed shisui..thats how itachi got mangekyou..remember that you can get mangekyou if you kill someone close to you..shisui is like a brother to itachi..and sasuke got mangekyou by killing his brother itachi.
the counter part in this theory is how uchiha shisui became a mizukage if kisame acknowledge tobi as mizukage.
edit : forgot to mention.. i think thats why tobi's sharingan is always activated because he is shisui and he is a uchiha..just like when itachi activates his sharingan constantly.
well anyway its just a theory..enjoy