Something inside that shell on the Juubi.
Firstly I would like to apologize if this has been pointed out.
I believe the next and final transformation will be a MUCH different Juubi than before, and perhaps smaller as well.
That shell on its back has stayed this whole time, the only thing not changing. and consider this panel
Notice when Madara says "should I give it more power too" it focuses on the shell and starts to rumble it looks like.
Anyone else think someone will "hatch" Kishi of course was influenced alot by dragon ball z.