Alan Gratz (a really good writer, whose books you should read) has a blog post envisioning what a crossover betwixt Twilight and Buffy the Vampire Slayer might be like:
Following up on the tip from Oz’s werewolf contacts, Buffy climbs in the window of recent Sunnydale High transfer student BELLA SWAN to discover EDWARD CULLEN, a vampire, watching the girl as she sleeps. Edward, apparent-age 17, is impossibly beautiful, with angular features and marble-like skin that sparkles.
BUFFY: Whoa. Turn it down there, Tinkerbell.
EDWARD: Shhh! You’ll wake my darling Isabella!
BUFFY: Right. Sorry. It’s just you really ought to take the batteries out. Somebody might mistake you for a Christmas tree.
EDWARD: I’m sorry. It’s my vampire skin. It sparkles in the sun or the bright light of the moon.
It’s great fun, and can be read at http://gratzindustries.blogspot.com/2008/12/edward-vs-buffy.html