Just to add to this, Jiraiya is out of the picture as well. Kabuto himself said that he was unable to locate the body when discussing people he would have loved to summon with Edo Tensei.Ok, lets clear some things up.
Edo Shishui - Fatal Error.
Even if he is summoned, he will not have his eyes; one of them was taken by Danzo and the other was given to Itachi. Plus it is an undeniable fact that those summoned by Edo Tensei, their bodies are exactly as they were the moment they died. This is why Itachi still has his eyes, plus one could notice that when Nagato was summoned, he had both his Rinnengan and his white hair; he got the white hair shortly before he died, so yeah. Edo Tensei summons someone's soul based on the last moments they spent on the world of the living, but as we all saw Kabuto could experiment on Madara's body to make him stronger, as Kabuto himself stated. And based on this one can say that Shishui's eyes or at least even one eye can be implanted. Ok, how? When both of Shishui's eyes are destroyed? Danzo's was crushed by himself and Itachi's was burned in an Amaterasu BBQ.
The Sage of Six Paths. - Another fatal error.
Now you can ask me why, and I will respond with ''Because --as someone already stated before me-- the sage died quite a few decades, even centuries, ago.'' Then one can tell me that the sage's DNA can be extracted from the bones of his --as someone already stated before me--; True, but it is a mistake again. Why?
First, how will someone know where the sage was burried?
Second, lets say his grave is found, will there be any bones?
Third. lets say that there are bones there, can the DNA be extracted?
Fourth, lets say that with some miraculus attempt the DNA is extracted, do you think that the sage can be restrained by a technique like Edo Tensei? The very man that was worshiped like a God will be held back by some child's attempt? (Compared to the sage, Kabuto is still a child.)
In my personal opinion, should the sage be summoned, the war will be over; I leave it to all of you to imagine how.
Orochimaru - Fatal Error again.
First of all, Orochimaru's soul is sealed inside the sword fo Totsuka and I do not think that Oro can escape from there. So even if Kabuto summons Orochimaru using the later's cells, it will just be a total zombie with no soul, perhaps even a vampire snake [XD] I am not sure about it. As for having Orochimatu's cells, well, Kabuto himself stated that he used the cells from the great white snake --Orochimaru's true form-- and implanted them on his own body, thus gaining Orochimaru's powers, but Orochimaru's cells tend to want to take over Kabuto's body. Last, regarding the small snake that was BBQed by the Amaterasu flames right after Orochimaru's soul was sealed and that being a small fragment of Orochimaru's soul that exists in the mortal plane... Fatal mistake. Even if a fragment still exists, which is highly unlikely, it can not do anything on its own and will never be able to become one with the rest of its existence.
But of course one can say that even by a small fragment, the soul can be whole again; after all Orochimaru had his arms sealed inside the Dead Demon's Seal, but I will say this ''Orochimaru still had over 80% of his soul, perhaps it was a 90%, and we all saw how sick Orochimaru was when the time to switch bodies came.''
Bottom line, Shisui and Orochimaru are out of the picture. The Sage? No, since he will be uncontrolable.
That was my opinion based on what I have seen so far in the manga, not the anime, and some speculations of my own. Hate me, detest me, flame me all you want, in the end I am here to express my opinion and I do not care about the names I will be called.
~The person behind the screen.
I'm convinced most people on these forums don't think.