I would just like to see him bring back the combination of SM and BM back or here. Kishi teased us with it and kind of told us with that power that he would have been able to destroy Nagato(not because he was weakened but because his chakra would have been mixed with Senjutsu Chakra which he would not have been able to absorb or see) but because mixed power of his own, Bijuu Chakra, and Senjutsu chakra he would be tooo powerful.
I don't know but I think that it is Naruto's last resort behind "THAT JUSTU", the Perfect Bijuu State, or the powers the other Bijuu gave to him..