Periscope's great, because of it I got to see about 2 hours of the last BF Day "live", the only issue is it can't be archived (or certainly couldn't then). We got plenty of great interviews with Colin, Michael Jayston, Matt Waterhouse and loads more thanks to Kris G.
Twitter questions are easy to pass off. I'm really hoping forum users who attend will ask questions in person in order to get something close to a complete answer. And another question for someone to toss out, Blake's 7, what can they tell and when?