It might be interesting if Guy was sorted to a house other than Dragonclaw, especially if his family were expecting Dragonclaw. Maybe he's sorted into Lionheart instead. OH! If you go with the RoS third series storyline, Guy might get sorted into the Wolvesheads House if he's Robert of Huntingdon's half-brother. That way he could be close to Loxley (I assume that's the Robin you mean), but also be really screwed up and conflicted. Why am I in this house when the Gisburne family has always been sorted into Dragonclaw? If you're looking for some Gisburne tension/angst, it would be great if he felt like an outsider -- and was maybe treated like one by the other students in the Wolvesheads House. You could have a friendship/eventual romance develop between Guy and Robin if Robin tries to befriend Guy -- or maybe they're thrown into the same dangerous situation together. There are endless possibilities!

You should ask grondfic what she thinks. She's a lot more knowledgeable about the Harry Potter universe than I am. She could probably give you some excellent advice.