Supernatural.

CARRY ON MY WAYWARD SOOOOOOOOON!

But seriously, I've know people have started writing this kind of crossover and then kind of left it hanging. I feel like it's a fun thing to have on just the Ghost Hunt forum.

There's plenty of ways you can go about this, you can have it so that Mai is related to Dean and Sam. Or, after Ghost Hunt, when Naru left, Mai left for America, got involved with Dean and Sam, and becomes a hunter.

You can have it so that BSPR actually workers with hunters, and that they deal with different level threats.

And SPR team member can be a natural witch.

Mai made a deal with a demon.

This is one of those stories where the clash of Supernatural's humor with Ghost Hunt's seriousness would come out with a very amusing outcome. You can stay true to cannon characterization. The only thing you would have to adjust would be the timeline. And if your a writer who likes to write various length stories you can have overarching stories with little plot hints, or you can have an entire chapter dedicated to a mission for tacos.

Like, going over into the Supernatural territory is that random.

You can also use this to take advantage of researching into different paranormal anomalies to better ground the story in.

Just, if you do this kind of crossover, do not recommend putting Sam and Dean on the backburner.