Look beneath the underneath:
Viz:
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The Bijuus are but one example
So what were the other creations with Banbutsu Sozo?
This:
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The Elder Son (Juubi's Yin) The Younger Son (Juubi's Yang)
All I'm seeing is theories built on theories?
1) Where was it stated that the Stone Tablet was Rikudou's?
2) The whole idea of Rikudou creating sons is ethically borderline barbaric, especially considering he was a monk.
3) The Shinigami's design was introduced in Part 1, the same time that Hiruzen repeatedly was stated to be Konoha's strongest Hokage and it was rumoured that the Uchiha shared blood with the Hyuuga, I doubt Kishimoto thought in detail about the plot ten years ahead.
4) Much of what was said was told by Obito, who was most likely told by Madara, who was most likely told by whoever etcetera etcetera, need I say more on that?
5) The whole Naka Shrine/Uzumaki Temple thing again is just theoretical, it doesn't suggest a blood relation, and could just as easily be interpreted as a way of safeguarding their use.