Naruto: Uchiha, Starting with the Mangekyō Sharingan

The Transmigrator: Uchiha Chinatsu opened his eyes and looked at the Mangekyo Sharingan in the mirror. “Though there’s no system, in this world where eyes rule, having this bloodline and these eyes, it’s not a bad start, is it?”

Mangekyo Abilities:

Tenma-no-Mikazuchi (God Emperor Birth Spirit): Can awaken and evolve his own bloodline, evolving the Mangekyo Sharingan into the Eternal Mangekyo Sharingan, unlocking the Otsutsuki genes within his body, and reverting from Uchiha to Otsutsuki.

Taka-no-Mikazuchi (High Emperor Birth Spirit): Can awaken and refine the bloodlines of others. Can allow Uchiha clan members to awaken the Sharingan, evolve to the Mangekyo Sharingan, increase the purity of the Hyuga Byakugan, and even evolve it to the Tenseigan...

This is a world of ancient background Naruto, with no fancy technological gadgets, no electricity or electronics, everything is so backward, feudal...

For example, polygamy.