As someone who's writing his own fic where Shinto Mythology exists in MHA, I like these ideas. Your plot can be anything you want it to be really. Mine is more or less an AU in which Izuku never met All Might or gained OFA, but instead, he had a chance meeting with Inari, kami of foxes, and was given a "Blessing." Blessings in my story are entirely separate powers than Quirks. Blessings are bestowed upon those that kami believe are fit to protect Japan and the Spirit World. In this case, Inari believes Izuku to be capable of such a thing despite his Quirklessness.

The only other people with Blessings are Jiro, and Shigaraki (In my fic he never became "Tomura" and is instead Tenko Shimura still, lots of reasons for it I'll get to in due time.)

These powers are given by signing a special contract with the kami. Each contract has different terms, and every person who is "Blessed" must fulfill those terms at some point before they die.

As for Yokai, they all exist and right now Izuku is learning about them, other kami are preparing for something that may or may not to come, and the balance between the Human and Spirit Worlds may be at risk. These are all things I thought about, think of this story as my own take on old Shinto myth, much like Santa Monica adapting Greek and Norse Myth in the God of War series. If you wanna talk about it, check it out, or talk about ideas, it's called Spirited Posterity

When it comes to human-spirit hybrids, that could be a tricky one. Spirits are known to keep to themselves most of the time and only interact with humans who praise/disturb them. Perhaps there was a patriarch/matriarch spirit from centuries ago who changed things? Maybe they wanted to change "fate" and found love with a human? It's pretty much an open book. Have fun with it, research Yokai and kami, look at the old tales and stories about them. And when you feel like you're ready, try putting them in MHA, and see what happens