An idea that came to my head, I can't stop thinking about, and now I'm trying to exorcise: the sharingan isn't "eyes", because that would make negative sense; instead, it's a parasite (with varying levels of symbiotic), that can hook into the optic nerves of anyone with the Uchiha gene.
*slams her fist on the table* THIS IS THE GOOD SHIT, OP!!!!!
Decreased seratonin levels (especially after psychological trauma) allow the parasites to proliferate, inducing an evolved state of symbiosis, at which point the parasite itself starts to take over serotonin production and use. Once they get to a certain stage, the parasites begin to divert and store the serotonin, and can instantly use it to promote a burst of bone mass, ie. the Susanoo.
I love this idea so much. T_T
How about: The parasite - once it’s grown large enough to induce the mangekyo - merges it’s frontal cortex with that of its host (hello mentally unhinged crazy town). Let’s say the eating of the optic nerve is an adapted behavior that increases as it evolves. When the optic nerve is purposefully cut, the parasite’s hindbrain/brainstem is the only thing left and it continues to function if it’s attached to another optic nerve by which to gather nutrients/neurotransmitters. The parasite is effectively dead, simply running on automatic when it’s transplanted. The transplant also deprives the remnant of the parasite’s brain (the one still merged with the original Uchiha) of nutrients until it withers away.
Sanity returns.
Happy endings for everyone except Kakashi because he was a sad panda before all of this eye nonsense.