This is why I'm reluctant to start watching Stranger Things, knowing that Millie Bobby Brown was at LFCC last year and I didn't meet her. I know that if I watch the show and like it, I'll forever be kicking myself.
So I reckon the first guest will be male, and famous for his work on either Doctor Who or Star Wars or Red Dwarf. Also a repeat guest; this won't be their first time. I figure that's a reasonably broad yet still somewhat specific guestimation!