So I was looking at them, both have the opposite eye covered.
They both are high level shinobi and both old. Madara is known to have lost part of his power and is weakened. I think the answer is obvious.

danzou is madara's brother.

yes they both have one EMS sharigan and one that's not.

root and the mist practice's are very similar so i figured there was a connection and while I was trying to figure out the connection i realized this.
also you have to figure how DID madara know what itachi pulled. he had to have someone inside, someone specific that would know that itachi threatened danzou (which could only be danzou).

their hair is similar and they both only use one eye..
and well simply put any theory about danzou being madara or something like that, all that evidence supports that its his brother.
and the fact that it makes so much more sense for it to be his brother than madara himself or just a friend.

i dont really need to get into detail since most have read every madara/danzou connection theory, so just take all that evidence and apply it here...
(really im just to lazy to type up and list all the reasons lol)