That last coffin
I re-read chapter 490 again trying to figure out who was in that last coffin. There are really two categories of people it could be:
1. People who are more powerful than Madara. Considering that Kabuto doesn't really know what Madara's real power levels are (and now that Madara has the Rinnegan) this is a *very* small list of historical people. This could also be people with particular kekkei genkai that Madara is particularly weak against. In this case, the exact person doesn't matter so long as they are more powerful than Madara is. Some people who we know are strong enough to have defeated Madara in battle are off-limits: Hashirama, Tobirama, and Minato.
2. People whose body or existence pose an existential threat to Madara or his plan. This could be like a weird trick, somebody who knows an important secret about Madara, somebody who has a special relationship with him, or somebody who was able to stop his plan. This could also be a person whose very existence reveals a secret about Madara. For instance, if we said "Madara's real body", just existing would reveal a secret about Madara that would hurt his plans. This would have to be somebody very specific.
It's either somebody with more power than Madara, or somebody with a secret edge over Madara. Here are two examples: In the first case, the person could be the Sage of the Six Paths, clearly powerful enough to defeat Madara in combat. In the second case, it could be the "Madara's real body" theory that people keep talking about. If it were Madara's real body, people like Zetsu would realize that Madara was lying about who he was, and Zetsu may stop helping. This would destroy Madara's plan.
Read 490 again. Madara looks scared. He's frightened, shocked, startled. He's about to attack Kabuto and stops in his tracks. And beyond that, Kabuto acts like he has power over Madara. Kabuto's trump card is so important that Madara "Cannot refuse" him. Cannot. No choice. No recourse. Read the text again:
Madara: What if I say no?
<Kabuto summons the coffin>
Kabuto: Did you really think I would extend my hand to you without a backup plan?
Kabuto: ...You cannot refuse me!
Madara: You madman...How did you do this?
Kabuto: Oh, you know. Relax, I haven't told a soul.
In that last frame, you can see Madara's eye is very wide. He's scared. This is big.
It's somebody that Madara is desperately afraid of. He's so scared that instead of fighting Kabuto he agrees to work together with him and offer to give Sasuke over to him after the war. We don't know what Madara's ultimate plans for Sasuke are, but I think Sasuke is pretty valuable. This is a big deal.
The line where Kabuto says "Relax, I haven't told a soul" is pretty telling. This suggests that it's a matter of a secret, not just a matter of power. Everybody knows that the So6P is more powerful than Madara. That's not a secret, and even if other people knew it wouldn't matter: Knowing that a dead person is more powerful than Madara only helps an Edo Tensei user. If Kabuto told Kakashi "The Sage of the Six Paths is more powerful than Madara", what can Kakashi do with that information? Nothing.
Considering that Kabuto treats this like a secret, I think we have to rule out the "power" options: The So6P and his children, other random powerful ninjas, etc.
However, if it's a secret kind of person, maybe Kakashi would take that information and be able to develop a plan. I'm only using Kakashi as an example person here, of course. If Kabuto says "I know a secret. I have Madara Uchiha's real body in a coffin. Therefore, this is not Madara". Somebody like Kakashi or Naruto would find that information very valuable: Suddenly they know that this person, Tobi, is not Madara and does not have the power that Madara was supposed to have (no EMS, for example). That would help them formulate a better battle plan. That's the kind of secret that Madara would be afraid of.
So let's start looking at a list of candidates:
1. Madara's real body. This really does seem like the best option, considering the evidence I point out above. I don't think I would bet money on it, but this does seem the most obvious. I don't know if Kishi will go with something obvious.
2. Obito: This doesn't make sense. Obito was not powerful, didn't know anything about Madara, and his existence doesn't reveal anything about Madara.
3. Izuna: Izuna has MS, so he would be a power threat to Madara. Of course, Itachi has MS too, and Madara isn't afraid of Itachi when Kabuto summoned him. Madara doesn't seem like the kind of person who would let his emotions get in the way of his plan, so I don't think he will go easy on his younger brother anyway.
4. Shuisui: Shuisui did have a unique hypnosis technique that Madara has expressed an interest in gathering for himself. This doesn't seem like something that would scare Madara though, or something that would force him to negotiate with Kabuto.
Another option is that the person in the coffin was somehow related to Zetsu, and was able to stop/kill/control Zetsu and therefore would be able to stop Madara's plan. I don't know who it would be, but Kabuto does seem to know a lot about Zetsu. Withou Zetsu, Madara's plan is dead.
So that's my analysis of the coffin. I offer two guesses, based on the information we have: Either it is Madara's real body, or it is somebody who hasn't been in the manga yet who is able to stop or ruin Madara's plans.