I think Fred and George Weasley would have made the Slug Club easily. It's obvious that though they're pranksters, they're talented wizards. Slughorn would have no doubt enjoyed a laugh or two at some point. Also, he had a talent for spotting the would-be famous people (Gwenog Jones comes to mind), and I believe he would have been able to foresee that the Weasley twins would run some sort of successful business in the future.

Another possible student could be Roger Davies. He's intelligent, since he's been placed in Ravenclaw, and a charmer (he got Fleur to go with him to the Yule Ball, didn't he?).

Candice//Gryffindor