I'll leave the Marada is Danzo stuff alone for now, cause I like the Fugaku theory better.
If Danzo is Fugaku then that would mean that Itachi was either part of Fugaku's Coup D'etas plan, or Fugaku saw it coming and used some kind of substitution tech to fake his death. After all, Fugaku was Itachi and Sasuke's father. Not as powerful as either, of course, as he didn't do all the questionable things that itachi and sasuke did to gain their power. But likely, just as intelligent as his sons. Itachi was still young, he could have been fooled.
Then again, Itachi may have known that Danzo was his father. If itachi was part of the plan then Fugaku (who would have also been posing as Danzo at the time) put Itachi up to the whole thing, even the part where he cut up his own father, leaving him almost dead so he could fake his death. Like Madara said, Itachi was just following orders. Fugaku ultimately had the same goals as Danzo. He wanted to be Hokage and take revenge on the Senju. Danzo hates the Senju ideals and wants to be Hokage. Now that we're finding out that Danzo may be an Uchiha, it kinda makes sense that he'd be Fugaku.
There's also one other Uchiha that Danzo could be, but the only thing there is to support the idea is Danzo's physical condition. Messed up right arm, crushed right leg, sharingan missing from his left eye socket... He could be Obito. They never collect Obito's body and we never see him die at the end of the Kakashi Gaiden. But I feel like kakashi would have noticed by now.