Another KaiShin AU that just crossed in my head :3
Villain X Hero AU but also soulmate AU!!
Here I think that Shinichi is the Hero, while Kaito is the Villain. I'm not sure about the storyline yet, but one scene I'm thinking about is Shinichi being anti-romantic. He would rather save the world than save his soulmate. Meh that sounds excessive, BUT. Kaito, who knows that he and Shinchi are soulmates, wants to burn the world down. Yep he is jealous.
Maybe they know they are soulmates from how if they are touched, they will feel burned inside them (?) Sigh I suck when it comes to soulmates.