I think Kishi wouldve mentioned Kagami alittle more if he was the man behind the mask(Tobi)... Shisui was mentioned alot.. Izuna was mentioned a few times, was on par with Madaras strength, he couldve just took someone else eye, if blind madara could take Izunas eyes, why cant Izuna take someone elses eye (could explain why Tobi never shows MS)?? lol. Obito was mentioned a few times also, since Kakashi has space time tech, Obito couldve had it too but very unlikely that he survived and became evil... The SO6P older son, mentioned like once, but still plausible.. So Izuna and the older son are the most likely candidates for the behind the mask mystery....