It just seems silly and arbitrary to me to insist that a motif can't omit aspects of a theme that don't fit what the producers what to do. How a motif is used should be dictated by the needs of the story, not by trivia. So there's not a Knight Fangire because the creators decided that the story simply didn't require that such a character or role exist.
Also, to be honest, I find it a bit appalling to argue that motifs can only be used in very literal ways. That's usually a sign of questionable taste and poor creativity.