Episodes with callbacks to the star's previous work are fun (example the Firefly refrences in Castle). One of my current favorite shows is Bones staring David Boreanaz. David also starred in the popular Buffy the Vampire Slayer featuring Sarah Michelle Gellar. I think it would be fun to see Angel and Buffy together again. I'm would love to see Sarah play a character convinced Vampires are real but in reality is just a serial killer.