Obviously, a Dexter/BtVS crossover, Post Chosen and Not Fade Away. Post season 4 for Dexter, so no Rita.

Debra/Xander pairing. They had a secret relationship, that's becoming serious, Xander decides to tell her about the real night life and brings Illyria with him to prove it. Plus Illyria has chosen Xander to be her new High Priest, after Not Fade Away.

They eventually meet Dexter and Illyria recognizes him for what he is, but keeps it to her self.

Go on from there but eventually, Xander and Debra have to get married.

If other Dexter characters know about ...

Your mission, should you choose to accept it is to have Xander Harris sent to New York to recruit a new Slayer, Alexis Castle the daughter of famous Mystery murder writer Richard Castle, who has more than a passing resemblance to Caleb.

Point 1: The story must begin in a Police Station.

Point 2: Xander must have assaulted Richard Castle without knowing that he's Alexis's father.

Point 3: Nobody told Xander that Rick looks like Caleb. Whether this was accidental or a practical joke on somebody's part is up to you.

Point 4: Caleb must be Rick's older brother. ...