In some interviews he has said he won't comment on whether we have seen the final Cylon. I thought he said we had already seen the fifth Cylon but I may have been mistaken, but he did say he has left us clues as to who the fifth Cylon is.

That makes it harder to be a totally new character, for those clues would have to be fairly generic. However, it could be an offscreen character like Joseph Adama, in that we already have one tricky clue "Adama is a Cylon."

However, I only rate Joseph Adama as a low-probability candidate. For example, Inner-Six has called Hera the first hybrid, and if she's real and a pipeline to the Cylon god, and not lying or talking about something else, that would rule out anybody who had a kid before Hera was born.

BTW if you have a link where he says the last Cylon does appear in Crossroads or that clues appear in Crossroads, please point us at it. Joseph Adama is, in a dramatic sense, "present" in Crossroads, but not in the flesh -- as far as we know. Though I have to say, since I wouldn't recognize any of my Grandparents if I met them at age 20 even though I have seen pictures, it's not out of the question that Joseph Adama could be a character in the show, though we're told that Apollo looks like him by Romo.