The surpise 6th coffin

1st
Entirely possible that the sixth coffin could have contained Shisui. Nagato's reanimation body retained his eyes even though Tobi/Madara had removed them. It is possible that Kabuto's "trump card" is Shisui's body that may retain a portion of his original strength. Although now Kabuto realizes that Itachi is beyond his control so it is possible that Shisui would also be beyond his control as well. Or perhaps the only reason Itachi is unable to be controlled it only due to the eye implanted within the crow.

2nd

sorry man but look where kabuto talks about the edo tensei jutsu, he says he can't revive shisui due to lack of DNA

3rd

For me... i think its Minato..xD
just by the reaction of madara..

4th
It can't be Minato because he sealed himself. We've discussed this so many times, but we'll never really know until, well you know.

5th
ahh...I was trying to think about the 6th coffin, but when you said that, I was just like "Ahh! Maybe that's it?". Kabuto doesn't have Shisui's DNA...just yet, right? He could get it from the eye of the crow, and in fact I think he is trying to do it now using Nagato. He did say his luck was getting better and better...

but then that would mean that the body in the 6th coffin isn't Shisui. The suspension is killing me~~~

and by the way, I don't seem to be able to recall what happened to Danzo's body, because Danzo did have one of Shisui's eyes, right

6th
my brother thought it would be madara's brother too, and i think it's reasonable guess, i personally think it might be madara's original body, i don't know why he would be afraid of his brother like he seemed to be afraid of the mystery body, but it might have also been shock not fear, there aren't a lot of clues as to who it is, so it's really hard to say with any accuracy

7th
Danzo destroyed the shisui's sharingan which means no one can use it ( it's jutsu etc. and has no light hope you guys understand ), but in shisui's sharingan ( which danzo had ) there is enough DNA to revive shisui, he might probably one of the main plot of this war ( tailed beasts, madara, kabuto and shisui's eye ).
-As for the 6th coffin after what i have seen about uchiha i mean what their eyes ( especially MS ) are capable of it won't be stupid to say it's madara's brother cause we don't know what MS power he has probably a jutsu that tobi's time space jutsu can't dodge.

8th
If it's the Rikudou, I doubt Kabuto would be letting Madara make all the demands. I bet the guy in the coffin is someone who will potentially sabotage his plan (most likely Izuna) but not so much that he'd let Kabuto call the shots. In fact, I doubt Madara cares if Kabuto exposes him now since Zetsu and Sasuke (somewhat) are on his side. Kisame's dead. Even if Tobi really isn't Madara, who cares? He's got the Rinnegan and is still stronger than everybody. Doesn't matter if he was some weakling before (he wasn't anyway). So pretty much, Madara and Kabuto are trying to outdo the other. The Rinnegan was Madara's goal anyway but it got rid of the 6th coffin threat probably. Now, apparently, Kabuto has some other ace (which he didn't tell Madara about). In response, Madara got the 6 Paths. Kabuto's trying to get Shisui's eye. Wonder what Madara will get next?

9th
By using the name Madara Uchia, Tobi was able to make everyone believe his threats have some validity (a single man declaring war on 5 nations isn't something an ordinary man can do). Since the 6th coffin was brought into the manga before the actually war started it is possible that it contained someone that simply knew the true identity of Tobi. The coffin may simply be someone with information rather than someone of equal or greater power.
As far as "if the 6th coffin isn't someone strong, why doesn't Tobi just kill them?". You need to remember that Tobi tried to avoid any actual combat because it was stated a few times that he is just a meer shell of the man he was and cannot afford to be weakened by fighting unnesessary battles.

10th
My opinion is that is is the uchiha clan ancestor. I think this because kabuto is not a stupid person and probably didn't summon the sage of the six paths because it would be too much power to realistically control. However, the uchiha ancestor has a eye jutsu that we have yet to learn about. It would not supprise me at all if this is who kabuto summoned and if his eye jutsu is written about on the stone tablet that the sage left behind. If this is so, then madara should know about it, thus explaining his WTF face.

11th
I think its sage of the six path