I would definitely love to read a Buffy/Yu-Gi-Oh crossover story that shows how Yami Yugi, Yami Bakura and/or Yami Marik would take on either Glory, Warren or Caleb some time. They would've definitely gone into shock at seeing how powerful Yami Yugi, Yami Bakura and Yami Marik can really be, especially when they put their minds to it as well.
First of all, for one thing, Yami Yugi's Shadow Magic would definitely make Warren learn about what being in slavery is about, and he would learn that the hard way since the Cerebral Dampener would never work on Yami. Instead Yami would make it work against Warren in quite the most intriguing way, of course.
Also, Glory wouldn't be able to use her brain-sucking powers on Yami, either, because he could probably turn it against her or maybe transform her into a little kid if he wanted to.
And finally, as for Caleb, he would have quite the battle against Yami on his hands, especially if Yami summoned the Dark Magician, Obelisk the Tormentor, Slifer the Sky Dragon or the Winged Dragon of Ra. But either way, these villains would've been sent to the Shadow Realm if they lost (which they definitely would).