My views. All of these guys mentioned seem to be very likely, though I did not read all the posts in this thread...
Sage of the Six Paths- Very likely. For one thing, to the guy who said that Madara wouldn't know what he looked like, it may not matter. When the 3rd fought Orochimaru, there were three coffins. Two that came up successfully, and one that was stopped. The thing was, the coffins had the symbol on the front of who it was. Also, the Rinnegan, or the necklace that the Sage and Deva Path wore stood out. On top of that, they put all this hype about the Sage of the Sixth Paths up and we have yet to see his abilities.
Izuna- Until he got his eyes plucked out, he was said to be Madara's equal. Now I believe he gave them willingly to Madara, because if Izuna was almost equal to Madara and still had enough sight left to combat before his eyes were taken, there would be no way a blind Madara could take his eyes just like that. This isn't Dragon Ball Z where you can sense an opponents movements to fight them. If Madara is actually a shell of his former self, then Izuna coming back to the story would serve a lot to the current storyline because he would most likely be brought back with the power he had before his death.
Madara himself- Well, I still think Tobi is Madara. It is possible that he is using a jutsu to take over someone elses body. We have seen it in the Naruto universe with Pain, except they were corpses that had chakra rods moving everything, but essentially it all comes down to using a body that isn't actually yours. It made me think a bit...Did Madara really use Izanagi against the first? When Danzo used it, his health remained the same minus him losing a Sharingan and his chakra taking a huge dip.
Hashirama- To the guy who said that he can't be resurrected because he is sealed in the Death God. That may be true, but if you watch the fight against the 3rd and Orochimaru, well not watch, but read the fight again, you'll see that Madara tried to summon someone who was quite obviously Minato. Minato had sealed himself with the other part of the Kyuubi's chakra, but Orochimaru was still able to bring his soul back.
The argument stands that Minato put some of his soul in Naruto, but it was actually life force and we have no idea if that is the same thing. Plus, do you honestly think Orochimaru knew that before summoning Minato?
It could be Hashirama, but I don't think it is. It wouldn't stand to have Hashirama appear like that again.