This might sound bizarre or unlikely but just listen up;
What if Naruto's Bijuu Chakra Mode looks like as it is not only because his using Yang Chakra, but because of the Eight Trigram Seal?
After the battle with Kurama, Naruto successfully captured his "Yang Chakra", contained it with Eight Trigram Sealing, then something happened to the seal.
IT EVOLVED!
It evolved into Rikudou Sennin's very own "Torii Seal", that Kurama even remarked belonged to the Rikudou Sennin upon seeing this fuinjutsu.
In my own expertise, Kurama's Chakra( any Bijuu/Juubi Chakra) combined with Uzumaki's main fuinjutsu created, or rather manifested the Rikudou Sennin's Chakra.
This is why Naruto's Bijuu Mode looks strikingly similar to Rikudou Sennin.
How come Bee couldn't access this form with his seal? Because its not Uzumaki Seal?
So it makes you wonder, was Rikudou an Uzumaki? Did he use Eight Trigram Seal or the Torii Seal to capture the Juubi?
I'll leave that to you
What if Naruto's Bijuu Chakra Mode looks like as it is not only because his using Yang Chakra, but because of the Eight Trigram Seal?
After the battle with Kurama, Naruto successfully captured his "Yang Chakra", contained it with Eight Trigram Sealing, then something happened to the seal.
IT EVOLVED!
It evolved into Rikudou Sennin's very own "Torii Seal", that Kurama even remarked belonged to the Rikudou Sennin upon seeing this fuinjutsu.
In my own expertise, Kurama's Chakra( any Bijuu/Juubi Chakra) combined with Uzumaki's main fuinjutsu created, or rather manifested the Rikudou Sennin's Chakra.
This is why Naruto's Bijuu Mode looks strikingly similar to Rikudou Sennin.
How come Bee couldn't access this form with his seal? Because its not Uzumaki Seal?
So it makes you wonder, was Rikudou an Uzumaki? Did he use Eight Trigram Seal or the Torii Seal to capture the Juubi?
I'll leave that to you