And tha'ts fine.

This tangent was started by my saying that the Sharingan was a bit too tempting to not go after after all... But since Senko has already demonstrated an interest in the Uzumaki Clan's vitality and would need the Senju clan to achieve her goal of mastering all forms of Senjutsu.

I was basically just setting up ground rules ahead of time since, well, if she gets what she wants then the Rinnegan is inevitable, and I was using the in-canon logic to prevent things from getting too overpowered.

Honestly, I was mostly focused on how the White Snake power's healing factor interacted with the Mangekyo(enough to undo the damage, but not fast enough to matter) and establishing that Senko's hypothetical Magekyo powers would be relatively basic support techniques instead of something overpowered and nigh-unblockable. The "she'll probably get the Rinnegan but it'll be weaker than Madara's because of these reasons from canon" was mostly an afterthought.

Honestly, in-character Senko would probably dismiss it as a freak mutation caused by an unintended reaction between multiple bloodlines. "No, this is not the Rinnegan. That's just a myth."

I could just as easily say that adding Kaguya Clan DNA to her genome and/or Hyuga to gain the Byakugan(which she wouldn't be able to do use at the same time as Sharingan) makes her count as an Otsusuki when combined with Senju and Uchiha traits so her pursuit of the tailed beasts' chakra later on eventually has the same effect as the cinnabar elixir and gives her her own unique Rinnegan with powers designed to be compatible with the game's power level.

As for deliberately modeling yourself on Indra and Ashura... It might work for Indra but it seems too power-hungry for Ashura. It's also dependant on their chakra not having already attached to someone since the implication is less "being spiritually similar to them makes you generate their power" and more "symbiotic force ghosts."