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The sun tomoe is an indication of six paths yang. I know its counter intuitive for an Uchiha to have the six paths yang power but it makes perfect sense in Sarada's case. Why? Because of the medical ninjutsu.
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Naruto healed Kakashi's eye and revived Might Guy using the power of six paths yang. That war arc was intense!
So I guess you are thinking if she has six paths yang why didn't she unlock a rinnegan instead of a mangekyou sharingan? I think it is because the rinnegan is symbolic of the cycle of death and rebirth. So the circumstances involving her rinnegan awakening will involve the death and rebirth of a loved one. Everyone seems to think Sasuke is dead when Boruto and Kawaki face off on the hokage monument. What if he really does die but Sarada revives him using the six paths yang power? This method of awakening is similar to the way Hagoromo unlocked his rinnegan and mangekyou sharingan.
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