1. Rinnegan is able to control the Juubi/Gedo Mazo.
I'm pretty sure if Rikudou had Rinnegan from birth the Juubi wouldnt be called a god, since it would be tamed.
2. Sennin Mode - Rikudou is said to discover chakra, normally this would take place in nature. Would also explain how he is able to seal the Juubi in the moon after gaining Rinnegan (Since Rinnegan has great chakra draw)
3. A Jinchuuriki inherits traits from the Bijuu
Its shown with Kurama especially, how Naruto got his whiskers and the ability to sense negativity, same could apply to Rikudou gaining Rinnegan after sealing the Juubi.
The Rinnegan you could say is the "good side" or pure part of the Juubi's eye. I believe the Juubi's eye contains loads of powerful dojutsu, including Sharingan, and Byakugan's. Sharingan for the obvious reason of the 9-Tomoe standing out, now 7-Tomoe because its incomplete.
Byakugan activated is shown having arteries stand out at the eyes, Juubi has something similar but just black. Since the Juubi is since to be the origin of everything.
I'm pretty sure if Rikudou had Rinnegan from birth the Juubi wouldnt be called a god, since it would be tamed.
2. Sennin Mode - Rikudou is said to discover chakra, normally this would take place in nature. Would also explain how he is able to seal the Juubi in the moon after gaining Rinnegan (Since Rinnegan has great chakra draw)
3. A Jinchuuriki inherits traits from the Bijuu
Its shown with Kurama especially, how Naruto got his whiskers and the ability to sense negativity, same could apply to Rikudou gaining Rinnegan after sealing the Juubi.
The Rinnegan you could say is the "good side" or pure part of the Juubi's eye. I believe the Juubi's eye contains loads of powerful dojutsu, including Sharingan, and Byakugan's. Sharingan for the obvious reason of the 9-Tomoe standing out, now 7-Tomoe because its incomplete.
Byakugan activated is shown having arteries stand out at the eyes, Juubi has something similar but just black. Since the Juubi is since to be the origin of everything.