Colonel Joe Kittinger would make a great guest.
He was, after all, the first person to go into space!
For shuttle astronauts, Winston Scott is a great speaker and is a famous spacewalker due to his EVA picture being widely promoted.
And not forgetting Boris Volynov - a popular guest at a much earlier Autographica in Northampton and a man with a great story to tell.
Also, some of the Skylab guys we haven't yet seen in the Uk would be good - Joe Kerwin, Owen Garriott, Bill Pogue or maybe Paul Weitz again?