Another possible Methos could be a woman, under the theory that whenever they lost Methos, it was simply because "she" had stopped cross-dressing. This would probably be ignoring a lot of evidence, but the same is true for most of the other candidates. I don't have a specific character in mind though.

I suspect that at one point, Daniel or Giles would try to calm everyone down and then suggest trying to get into Methos's head to predict where he is instead of assuming that he's on the show. I'm not sure whether they'd succeed or not, considering all the conflicting information flying around about Methos.