Madara and Izuna were the original wielders of the Mangekyou sharingan, since then we have only seen another 4 unlocked, 1 being kakashi's. This Doesn't mean others haven't unlocked it, but the ones that have been unveiled for a reason.
I an under the impression that the mangekyou is a more elite bloodline, as Itachi said to sasuke he has the potential to unlock it. So here is my theory.
Madara is the father of kagami, his only son. because he doesn't have a brother, the mangekyou would not awake, because it didn't have potential to become EMS. Kagami then went on to have two sons, Obitio and shisui, people have made this connection before and I am also a believer.
Izuna had a son, who was either the father of fugaku or fugaku himself. he then went on to have Itachi and sasuke.
Now, I think Izuna was betrayed by madara, which in turn led to the cycle of hatred that unwillingly gave birth to the EMS, a brother willing to take his own brothers eyes.
Tobi, I believe to be an Uchiha directly related to Madara or Izuna, So in my opinion he is one of wither kagami,setsuna or Fugaku.
I an under the impression that the mangekyou is a more elite bloodline, as Itachi said to sasuke he has the potential to unlock it. So here is my theory.
Madara is the father of kagami, his only son. because he doesn't have a brother, the mangekyou would not awake, because it didn't have potential to become EMS. Kagami then went on to have two sons, Obitio and shisui, people have made this connection before and I am also a believer.
Izuna had a son, who was either the father of fugaku or fugaku himself. he then went on to have Itachi and sasuke.
Now, I think Izuna was betrayed by madara, which in turn led to the cycle of hatred that unwillingly gave birth to the EMS, a brother willing to take his own brothers eyes.
Tobi, I believe to be an Uchiha directly related to Madara or Izuna, So in my opinion he is one of wither kagami,setsuna or Fugaku.