I utterly doubt it will be the Sage of Six Paths. I wouldn't doubt it if it was his brother. It will probably be either an Uchiha, Orochimaru or if anything the Yondaime Hokage. Or if anything anything Yagura.I was thinking it was his brother's coffin since I'd be pretty surprised if Izuna wanted revenge against his brother. It could also be the Sage of Six Paths or something.