Re: the last man in the coffin kabuto showed madara was???????(warning major spoiler)
Here is who I believe might of been in the last coffin...
Madara, there was a lot and still is a lot of speculation that Madara was able to cheat death by changing bodies. If Kabuto could find his original body he could use it.
I don't do research and I don't pay attention to interviews with the writters, I make bold assumptions off the manga and show.