So I have heard a lot opinions about the 6th coffin like Sage of 6 paths or Jiraiya or Izuna Uchiha or Uchiha Shisui e.t.c.
Most of the guesses are wrong because in the manga it is already stated that Jiraiya and Shisui cannot be brought back with Edo Tensei each one for his own reason.
Jiraiya His body is in the depths off the see and it cannot be reached.(IMO the only guy who could reached it was Kisame but he is gonner)
Shisui Uchiha his body has dissappeared and Madara said that his eye which belonged to Danzo is destroyed and the DNA left is not enough.Also the second eye was destroyed by Itachi (Amaterasu).
The guy in the 6th coffin must be someone a lot important and strong because his revival was a good threat for Madara to accept Kabuto's offer for co-operation.
BUT after Madara got Rinnegan he said to Kabuto that he would kill him if the latter did not explain how Edo Tensei works.So the power of Rinnegan made Madara to forget about the coffin.
The possibility of being Izuna Uchiha is very low because Madara with EMS and without Rinnegan was already stronger than his brother so he would not be afraid of him.
The theory of Sage of the 6th paths is kinda lame IMO.
Also in the 6th coffin may be the body of the real Uchiha Madara or an other character who is unknown in the series.
- Spoiler:
What do you believe?