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Kabuto says "I will learn the secrets of the sage of six paths." But wait a minute, if he was able to summon him why not just control him and discover his powers that way?
Most peoples theories about the sixth coffin have been based on the belief that Tobi is Madara. If so, then the last coffin must be Izuna. He is the only person who has ever been able to match his brother's fighting strenght other than the 1st & 4th Hokages(Neither of which can be summoned since they are sealed in the belly of the Death God).
If you don't believe that Tobi is Madara, then the sixth coffin must contain someone who knows the true identity of Tobi or MADARA himself!!!
FYI I'm leaning towards Izuna. I believe he forced Kabuto to tell him the secrets of Impure World Ressurection so that he could stop the jutsu in order to avoid having to fight the only person who was considered his equal other than the 1st Hokage.
You're right about the Hokage, Kabuto cannot summon any of them (Except Tsunade, after she dies). I don't know if he could summon Danzo, although I don't see why he would want to.
You're also right about the sage. If Kabuto had the ability to summon the sage, he would do it and use it as an obvious show of strength. He hasn't done that. Also, like you mention, he could just ask the Sage any questions he wanted directly, and not need to do sneaky things to learn about them. Also, when Kabuto is listing who he cannot summon, he specifically mentions that he could not find the body of Shuisui or Jiraya, and that he had an idea of where to get the sage of the six paths' DNA, but didn't have it yet. That would have been unnecessary if he had already summoned the sage.
On another point, why would the sage have been particularly frightening to Madara? Nagato has the rinnegan, 4 sacred tools were obviously in Kabuto's possession to give to the Kin/Gin brothers, and Madara has 7 of the tailed beasts that the Sage got most of his chakra from. The sage might be wise, but Nagato and the Kin/Gin brothers would have been just as powerful without the juubi, and Madara has 7 tailed beasts to use if needed. It isn't the sage, and it isn't a matter of battle power.
The last coffin might contain Izuna like you mention, but your logic on that point is flawed. It has nothing to do with strength. I think that Nagato is more powerful than Madara, but Nagato wasn't in the sixth coffin and Madara didn't seem scared to see Nagato. And even if Nagato wasn't more powerful by himself, the team of all former Akatsuki members together would be. Madara didn't seem to be afraid of the whole team, even when Kabuto summoned all of them first. This isn't a matter of strength.
Also, we saw the fight of Madara against Konan. Konan isn't one of the most powerful members of Akatsuki, but she laid a trap that almost killed him, destroyed one of his arms and caused him to sacrifice one of his precious sharingan. If Konan can do all that, imagine what Nagato could have done, or Nagato and Itachi together (with Kabuto in the background), or all former Akatsuki members could do. Madara is just a shell of his former strength, as Itachi mentioned at one point, and Madara said so himself as well. It doesn't require a powerful Uchiha to defeat him, since he isn't at full power.
And even then, Izuna only had MS, not EMS. Izuna wouldn't have had the ability to counter Madara's space/time jutsu. Izuna doesn't have more power than Itachi would, for instance, but Madara wasn't afraid to see Itachi appear in the coffin. In either case, Madara didn't even know that the summons kept their unique features until Kabuto told him that much later.
Recap:
1) When the sixth coffin appeared, Madara was already looking at 5 other coffins containing 5 very powerful former Akatsuki AND he was facing Kabuto. Those six fighters would have been very powerful together, but Madara wasn't afraid of any of those.
2) Kabuto mentioned that he had several summons as well. Madara didn't seem afraid of that either.
3) Madara would not have been afraid of a person with MS eyes. First, he didn't know that the summon creatures even kept their unique dojutsu (Kabuto didn't reveal that detail until later), and Itachi was in the first group of summons, but Madara wasn't afraid of seeing Itachi.
4) Arguably, Nagato and probably Itachi were as powerful or more powerful than Izuna. It's hard to compare, but Itachi was a genius among the Uchiha clan. Nagato is definitely more powerful. Kakuzu might be as well, he was very strong and fought against the first hokage without dieing.
Again, I'm not saying Izuna was not in the coffin. If he was, Madara was not afraid of him because of strength (there were other summons to be afraid of) and not because of unique MS abilities (Itachi also has MS, and Madara didn't know that the summons even had or could use their MS). Actually, looking at these facts together, it doesn't make a lot of sense for Izuna to be in that coffin anyway. There's nothing about Izuna that Madara should be afraid of.
Here's a list of people that we know it is
not, from things that Kabuto said, from the list of summons we've seen fighting already, and from the people I mention above that do not make any sense:
1) People we've seen summoned already (old akatsuki members, old kages, old jinchuuriki, other powerful ninja)
2) People who are sealed and cannot be summoned (old hokage, maybe Orochimaru)
3) People whose DNA could not be found (Jiraya, Shuisui, Sage of six paths, the bodies for Pain's six paths technique)
It also doesn't make sense to be Izuna (probably can't find the DNA, has no unique skills that Itachi wouldn't have, is not more powerful than any of the other summons).
I don't have a theory for who it could be, but I know a lot of other theories don't make sense. We are slowly narrowing it down.
