I'll have to say John first since we have a common interest: Music. Granted the type of music he might be into might be different than mine but I've learned to appreciate the old stuff a long time ago and find that some of it is actually much better than the newer stuff. He does have kind of a "connection" to some of hard rock's finest though. I'm sure you all remember the episode "On the Road Again" right? Teddy "Zig Zag" Andreadis played Mongo. About the last thing Andreadis did prior to that episode was tour with Guns N' Roses on the Use Your Illusions tour in which he played keyboards alongside Dizzy Reed and harmonica (on one song: "Bad Obsession"). I recall his Wikipedia entry saying that he owns a club where John as well as Slash makes some appearances. They jam I think. John would know all about any kind of music I think. I'd have a blast talking to him about that.
I had the pleasure of chatting with both Dave Coulier and Jodie Sweetin years ago. Well not in real life. You might remember Jonathan Taylor Thomas's Celebrity Sightings. They had a paid membership and regularly had chats with people like Jodie and Dave. They had kind of a tradition of having an "after chat chat" in which said celebrity actually chats among the fans regularly. Meaning that instead of the moderator just funneling the question to the celebrity, the celebrity just chats as we would. I didn't get the chance to talk to Dave one on one but they did pick my question. I asked him who he'd tour with. He said he'd love to tour with Aerosmith. Sometime later he said he didn't think he'd be touring with Alanis Morrissette anytime soon. LOL Someone asked him about "You Ought To Know" during that chat.
I did however get the chance to exchange some words with Jodie directly. We ended up talking about music. Turned out she was into the Offspring at the time and her friend played "the fly guy" in the "Pretty Fly (For A White Guy)" video. I thought that was awesome. The impression I got of the both of them were that they were pretty nice. Dave came off as quite teddybear-esque.
Next up would have been Mary-Kate and Ashley. Since we're talking about Full House, I'll do this two ways: during their time on Full House and in the present....
In '94, given their many interviews, I would have guessed they were much like me when I was their age at the time. They were sometimes tagged as being unresponsive or a little unapproachable at first because they were shy but would become really friendly after they got comfortable with you. I sometimes like to imagine what a conversation with them would have been like between them and I. Nowadays they both seem like they have much more confidence and aren't as shy as they used to be. They both seem like they'd be pretty sweet in person and shockingly normal as some people have put it.
Hey if I had the opportunity to meet any of the cast members, I wouldn't turn it down.