I agree for a few reasons that it could be Izuna. For all we know, it was just assumed that he died as we just saw a flash back of his dead body when Tobi was telling the story and Tobi is a known lair and why else would he freak out about the coffin that contained the real Madara in it? They were said to be close to equals and the glimpse of his face we saw, though not much, and his knowledge of old/ dead people and all the villages and knowing about war, suggest he is old and had a lot of experience. He did grew up in a violent era.
Isn't it weird that Tobi and Itachi only brought up for Izuna once, who supposedly skilled but never heard about again or resurrected with other edo tensei but the real Madara was? Not like Kabuto couldn't do it if he dug up the DNA (only few exceptions it seems. There has to be a reason for that or why even bother bringing him up at all? He also seems obsessed with collecting/stocking good eyes and Izuna supposedly gave his to Madara.
Madara said Izuna was dead. crying
Didn't Pain also say that "Nagato" was dead? =/
These seem like pretty good ideas to me.
Honestly, whoever it is, I'd assume it's someone who we'd least expect and that's all the fun of the surprise.