Izuku is thrown into the Highschool DxD universe with Eri. They end up near the Yokai Capital, where a group of rogue Fallen angels are attacking the Yokai, Kunou is running from a few of them. Izuku stops them and protects Kunou, all while smiling, as he does.

Izuku x harem: Yasaka, Kuroka, etc. Ravel.

Father Izuku to Eri and Kunou.

One for All is stronger then Boosted Gear and Divine Dividing. Immune to Divine Dividing.

Yasaka has Izuku stay in Kouh, arrives day after Issei is killed by Raynare. Mittelt joins Izuku Harem.

Izuku stays human, if he does change races, he's adopted by Amaterasu. Becoming a Demigod... his smile will finally make sense. But then his costume will be weird, considering it looks like a rabbit, the avatar animal of the moon is a rabbit, but he'll be the adopted son of the sun goddess, which avatar is the fox.