Tobirama is an unknown variable since he only displayed like two Suiton ninjutsu back during P1 and nothing else but Hiruzen's performance, in comparison to everything that has occurred in P2, was honestly quite pathetic. The fact that he was returned to the world in the decrepit state he died in will probably not assist him either. Only Minato and possibly Hashirama if he was somehow unable to release himself from Orochimaru's control pose any significant threats at the moment. That could very well change though as power scaling will inevitably bring the 2nd and 3rd Hokage up to a comparable level.
Hiruzen's 'decrepit' state is irrelevant since his biggest obstacle back then was stamina but in his zombified state stamina isn't even an issue. Tobirama is the Hokage that invented the worst mass-destruction jutsu of all time--the Edo Tensei--so I bet he's trouble. And I don't think no one is strong enough to beat a Hashirama + Minato combo ("any significant threat" is an understatement--more like "world-destroying threat")
Ehhh, it's all just speculation though. But I don't think Sasuke is gonna be brave enough to announce such intention considering folks like Naruto and Bee wouldn't be happy with it, if he doesn't have a very significant card as backup.
Although I don't understand why Orochimaru healed Tsunade... Maybe, a favor? Or maybe he wants to ensure Sasuke doesn't achieve his ambition since it means he will be unable to "master all Jutsus"? But then again if his goals is against Sasuke then the whole Edo-Kages vs Current Kages is moot....
Bahhhhhhhhhhhhhhh Kishi. Just give me NaruSaku already!