I think it could easily be Uchiha Kagami for the following reasons.
1- His hair matches.
2- His time/space ability makes a lot of sense being that he was a pupil of the 2nd Hokage who we know was knoen for his space/time ninjutsu.
3- His knowledge of edo tensei being able to be used by the 2nd. As a pupil of the 2nd it makes sense that he would know what the 2nd could do.
4- Look at the panel with Sarutobi saying, "We've driven..."
Notice who is not with him and who is with him from the group of the 2nd Hokage's pupils. Homura, Koharu, and Torifu are standing alongside Hiruzen. Kagami and Danzo are not. It makes sense that Danzo wouldn't be there because he was probably inside of an Orochimaru test tube. Kagami doesn't have a reason to not be there except that he's an uchiha...you know, the clan of people who hate konoha?
5- He is old enough to have known Madara after his battle with the 1st and to make a plan with him.
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