19 Sharingan in Kekkai Sensen

Before we go in, yes I'm aware that everyone and their grandma has done a Sharingan in another world. I don't care, I'm fucking using it. It's a very easy power to write about, since so much is known about it, and it's easily recognizable. I never watched or read Naruto, and I still know what it is. Plus, there is an eye-related ability in Kekkai Sensen, so whose to say that no others exist?

I'm pretty sure everyone has the whole structure of progression for the Sharingan down by now, but just in case. Sharingan has two abilities baseline: Eye of Insight and Eye of Hypnotism. Eye of Insight allows one to see chakra and grants incredible eyesight and memory. The Eye of Hypnotism allows the use of Genjustu: Sharingan, which can be used to control the actions of others, even Biju. The Sharingan can evolve into the Mangekyo Sharingan if the user is subjected to the harsh mental trauma of loss. The Mangekyo grants unique abilities, such as Amaterasu, Kamui, Kotoamastukami, and Tsukuyomi, at the cost of eyesight. It also allows the creation of Susanoo, and with a pair blood-related Mangekyo can evolve further into the Eternal Mangekyo Sharingan, eliminating the side effect of blindness, though the eyes can still bleed. These eyes can also still evolve in Rinnegan, using the chakra of Hagoromo Otsutsuki.

As for plot, again reincarnation or transmigration are the only things that make sense. It's really hard to break away from these established isekai routes. But, let's be honest, it's a really easy way of explaining how an MC arrives in an fictional world. Well, I suppose you could have a random Uchiha be transferred to the world of Kekkai Sensen, but he'd probably have to be young, and at that time most likely won't know how to cultivate his chakra.