im betting on the fusion-solution. or more specifically: both uchiha and senju are decendants of Rikudousennin. Now that both an Uchiha and a Senju are close to death, yet demonstrate the will to go on, their will unites into that of Rikudousennins, resulting in the creation of a new biiju. Itīll be the Serobii, the 0-tails, due the relatively small amount of power Naruto and Sasuke feature.

During its creation, the Serobii is split in two, one half sealed inside Naruto, one inside Sasuke. Thus, both surviving their otherwise fatal blows. The Serobii proves to be some super cheerful, funny monster, to show the contrast to the cold, gruesome monster the Jubii is. And from here, the battle continues ...