Tharme stays but Raven and Marx gone. Raven was the backup, Marx could have been considered, but Tharme is too inexperienced for next year. I think this indicates either three at the back (not likely surely) or Ricketts wants a ball player in the middle of defence.
You could argue we weren't very good at starting attacks and the central defenders often get the ball first. Our two aren't the greatest ball players. I think we're looking for a classy central defender.