I have never been disappointed with a Avantasia release, hopefully this new one won't be an exception. And for the record, I prefer The Mystery of Time over Ghostlights.
As to who might appear on this new album, probably the usual suspects. I would also place my bets on Andi Deris appearing. According to this one interview, Deris and Kiske have become good friends during the Pumpkins United tour. I wouldn't be surprised if Kiske pulls a few strings with Tobi, since he is surely going to be part of it.