I think it's Shisui but what is confusing me is why Oro said to Sasuke "you're still a child???
Exactly. Izuna doesn't really hold any relevancy to the story other than being Madara's brother and having given him his eyes. His role in the story pretty much ends there. As for shisui, I don't think so for the reasons i stated previously.It should be someone from the time of the massacre and not Izuna. That would be a different generation. Sasuke wants to find out why Itachi protected the village and what a shinobi is. That part is the most exciting part of the whole story, by far.
What does eyes have to do with anything he has a mouth and a tongue, he can talk about what happened in the past that led to the coup and assault on the UchihaIdk now that shisui doesn't have his eyes.......so it can't be him.
Maybe some random uchiha