Another reason why it could be Tobirama Senju. Tobirama and the 2nd Raikage formed an alliance, however, Kinkaku and Ginkaku attempted a coup d'etat and left him near dead. Kinkaku and Ginkaku were sent to capture the Nine Tails but were eaten and gained some of his powers instead. Later on Tobirama was seen surrounded by twenty Cloud elite ninja known as the Kinkaku Force. Tobirama volunteered himself to be a decoy while the rest of his squadron escaped.
Is it not possible that during the coup d'etat Tobirama actually plotted with Kinkaku and Ginkaku? Sometime after that, the Kinkaku Force, lead by Kinkaku possibly, "defeated" Tobirama. Is it not possible this was the plot planned with Kinkaku and Ginkaku before? He created Edo Tensei which he could revive the Uchiha for his purposes. Now this is because he plotted the Uchiha massacre from the beginning. With them dead, he could revive them for his own methods. With Tobirama considered dead, no one would suspect him.
Also, Tobirama before his real death, could have plotted with Orochimaru, teaching him Edo Tensei. Then, he could have plotted for/known Orochimaru to revive him. Since he is the original creator, he must have known how to severe the control over his soul.
There are also many other parts of the theory that can be deciphered later on.
It could also be a member of the Sarutobi Clan, more specifically Sasuke Sarutobi.
The Sarutobi clan is known for high-level techniques, according to Naruto Wikia, but this is justified through Hiruzen's Jutsu, Konohamaru using Rasengan young, and Asuma to some extent. Sasuke Sarutobi was renowned as a great ninja, so great Sasuke Uchiha was named after him.
Now, theory time. Though the Sarutobi aren't exactly the founders of the Leaf, they are a great contributor to it. If I recall correctly, the Leaf had a certain hatred toward the Uchiha, going as far to keep them under watch. Now we know the 3rd was against this hatred. When he fought Orochimaru, I don't remember if this was disproved already, but it might be possible that the 3rd coffin was in fact Hiruzen's father Sasuke.
Forgive me for this "similar name theory", rather this "congruent name theory", but Sasuke UCHIHA might share a name with Sasuke SARUTOBI for a reason other than desired greatness. This would be the ultimate plot if you think about it. Antagonizing the Uchiha clan when the real enemy is a Sarutobi clansman.
Now how has Sasuke Sarutobi lived so long? Is it impossible that he used methods of revival similar to Orochimaru? Edo Tensei has 3 users so far. Kabuto, Orochimaru, and its creator, Tobirama Senju, otherwise known as the Second Hokage. Before we continue with the rest of the theory, let's explain Tobirama a little further.
Tobirama created many functions of the village. These include the Ninja Academy, Chunin Exams, and more importantly, the ANBU and Leaf Police Force. Why are these two important? The 4th Raikage stated that Danzo and Akatsuki might have plotted 3rd's assassination. This might be a stretch considering Danzo has control of ANBU Root rather than Anbu. But it is something to consider. As for the LPF, it was suggested by Tobi himself that it was a method to distance the Leaf from the Uchiha rather than a gift of good will. He also was the leader of the 3rd and the his two advisor's squad.
Anyway, back to the theory. Sasuke Sarutobi was a great shinobi, the father of the Third Hokage, and equally important: alive during the reign of the 2nd Hokage. Now it can be gathered from the Sarutobi clan members that they are devotees of the Will of Fire and believe strongly in the teachings of the Senju, specifically the 1st Hokage. It's possible that Sasuke Sarutobi believed in his teachings so much that he despised the Uchiha to a greater extent. Now, it is valid to say that Sasuke Sarutobi has great influence in the village, enough to have Tobirama use Edo Tensei on his dead body and then severe his control from it. Sasuke Sarutobi, with his "immortality" could have learned Edo Tensei. He would then portray an Uchiha, specifically Madara, and release the Nine Tails to attack the Leaf, putting blame on the Uchiha further.
Now with that done, he would later on, with help from someone else, maybe Danzo, spark the plan to assassinate the Uchiha. Posing as Madara still, he would assist Itachi. With these dead bodies, he could revive them for their powers and also to frame the Uchiha further, something along these lines. Somewhere in the mix he would want to control the world or something and thus have it so Itachi didn't have to kill Sasuke. With Itachi and Sasuke alive, there could always be blame on the Uchiha as long as they lived, that's why he assisted Sasuke so much.
tl;dr: Sasuke Sarutobi could be Tobi.