There have been many theories about who might be in the 6th coffin and I know this may sound crazy but i have a feeling i know who it is. I have reason to believe the person is the First Mizukage.
1. Madara ruled over the Mist Village for some time as stated by Kisame.
2. Madara controlled the 4th Mizukage
3. Blood Mist village is finally free from Madara's Control
4. The Second Mizukage has shown to be pretty strong and skilled even if he is an edo-tensei.
5. Almost all kages have been shown in this war like Raikages, Tsuchikage, Mizukage, Kazekages. Hokages have been revealed since the beginning of Naruto
6. Mist village is rumored to be birth place of Akatsuki
7. Mist village is not known to have participated in of the 3 Wars.
8. The 7 Swordsmen who where from Mist Village are edo-tensei.
9. Kisame was part of Mist Village and Akatsuki.
10. The Mist technique orginated from mist village.
Conclusion: The Mist Village has a lot of things to do with this story and Madara. Many questions still need to be answered as to why Madara decided to control the Mist Village. When he took it over he already had two tailed beasts in his possesion. The Second and 5th Mizukage have kekkei genkai which are pretty strong who's to say the 1st Mizukage didn't. Maybe during the reign of the third mizukage, Madara might have noticed that the Mist Village was becoming weaker and decided to go after it but he didn't maybe due to the fact the first mizukage was still alive and could have easily stopped Madara at any time. Madara must have waited and eventually the third mizukage and 1st mizukage die. He controls the Mist Village for some time. 1st Mizukage might have left something for Mei which could be her granddaughter in case Madara tried to take it over. Mei might not have noticed right away the 4th was being controlled. Madara leaves the village and peace is restored to it. Kabuto revives someone that Madara fears which could be the 1st Mizukage as he knows Madara's weakness. This is still just a theory so leave your opinons.
1. Madara ruled over the Mist Village for some time as stated by Kisame.
2. Madara controlled the 4th Mizukage
3. Blood Mist village is finally free from Madara's Control
4. The Second Mizukage has shown to be pretty strong and skilled even if he is an edo-tensei.
5. Almost all kages have been shown in this war like Raikages, Tsuchikage, Mizukage, Kazekages. Hokages have been revealed since the beginning of Naruto
6. Mist village is rumored to be birth place of Akatsuki
7. Mist village is not known to have participated in of the 3 Wars.
8. The 7 Swordsmen who where from Mist Village are edo-tensei.
9. Kisame was part of Mist Village and Akatsuki.
10. The Mist technique orginated from mist village.
Conclusion: The Mist Village has a lot of things to do with this story and Madara. Many questions still need to be answered as to why Madara decided to control the Mist Village. When he took it over he already had two tailed beasts in his possesion. The Second and 5th Mizukage have kekkei genkai which are pretty strong who's to say the 1st Mizukage didn't. Maybe during the reign of the third mizukage, Madara might have noticed that the Mist Village was becoming weaker and decided to go after it but he didn't maybe due to the fact the first mizukage was still alive and could have easily stopped Madara at any time. Madara must have waited and eventually the third mizukage and 1st mizukage die. He controls the Mist Village for some time. 1st Mizukage might have left something for Mei which could be her granddaughter in case Madara tried to take it over. Mei might not have noticed right away the 4th was being controlled. Madara leaves the village and peace is restored to it. Kabuto revives someone that Madara fears which could be the 1st Mizukage as he knows Madara's weakness. This is still just a theory so leave your opinons.