After the latest chapter i've been wondering who could Tobirama possibly want to bring back that he would create the Edo Tensei technique. Don't get me wrong here but Tobirama's characteristics don't suggest him to be the kind to sacrifice others for his own causes. Then i remembered him saying he's seen the Uchiha's suffered by their own loss of loved ones . . .could it be Madara's brother? or even his own? it might even be possible he tried to summon the SO6P to end the war (highly unlikely though.)
I'm just saying that Orochimaru was so driven by the grief and loss of his comrades and loved ones that he turned to using this forbidden technique so what then would instill Tobiramu Senju to create such a technique?
The waring period and lost of many clansmen? His Younger brother Itama? His Older Brother Hasirama? (its suggested he was alive during its creation) The SO6P? Madara's brother? His Father?