Remember Kurama and all the other Bijuus' past? what Rikudo Sennin told them? Naruto will lead all of them down the right path, which means... Naruto will get the Juubi soon. He has all of the fragments of their chakra. I bet if he uses Bijuu Mode + Sage Mode, his Bijuu Chakra and Nature Chakra will then break the Juubi free and it will become his bijuu.