I like the Steve Buscemi idea. He's just gotta be one of the best actors, at least in my book.
Well, my favorite guest-star for anything has already been on Stargate several times: John de Lance. He's just so great playing those evil roles, he's pretty good at good-guy roles, too.
Hmm, peeps from Trek, well I gotta say Roxanne Dawson would be cool.
Jerry O'Connell would be funny as some off-worlder.
Not too sure about Harrison Ford, tho. Somehow he just doesn't come off right as a prospective guest-star.