After having James as my first guest at the first Hallowhedon and him being such a good sport after his injury I would LOVE him to come back for Hallowhedon 6! And get to meet him properly.
A Doctor Horrible or S.H.I.E.L.D guest would be a nice change from the mostly Buffy or Angel guests. Would love Felica Day, Amy Acker, Sean Mayor, Amber Benson was so great last time I'd love her and Adam to come back.
Also don't worry about getting to many guests. Quality over quantity. Would rather have a few really enthusiastic guests (however big their role) than a load of boring ones. Perfect e