[Fran~]

I've thinked several things about that...

the last thing i thought was that Madara isn't Madara but an Uchiha... it could be Shisui, Sasuke's dad, etc... and the coffin is THE real Madara. But that would leave zillions of plotholes.

I also thought in your post as a possibility too.

There is also the chance that the coffin could be some hax ninja, why not rikuudo senin?.

It has to be something that Madara would really fear.