Personally I would love to see Billy as a cylon just to frak with Roslin's head. What a head job that would be! However I don't see that happening since (as it was mentioned previously) I don't think the actor would commit to a long term contract. /boggle
I would also LOVE to see the character of Ellen Tigh return as a cylon, simply because I loved this character and the way she was played. HOWEVER having said that, I think it would make her death less meaningful.
I wouldn't be surprised if Gaeta is a cylon, Starbuck or any of the "secondary" players (Doc Cottle, Dee, any of the "ex-nuggets", etc)
However I DON'T think Baltar, any of the Adama's, Helo (Sharon would have seen him at the "meetings"
), the Chief or Tigh are cylons. They each have an intertwined history with each other or notable history within the miltary/government that can be proved otherwise (exceptions out of my other list being Starbuck and Doc Cottle). Everyone else is too involved (longer than 7 years) within the "establishment" and therefore would be harder to be part of the cylon evolution into human form. At least...that's the theory I'm working on this week. LOL