A few types I think we could use:
Obviously the most pressing need is a strong and reliable inside midfielder. He'd be able to win centre clearances consistently even without the help of a dominant ruckman, since we don't have someone that dominates the hitouts to advantage like Gawn or Jacobs. He'd be equally good at stoppage clearances around the ground. Critical to our game-style is having players that work defensively as well. Ideally he'd be in the top tier in the league for tackles and defensive efforts.
Also a strong mid-sized marking forward to support Franklin. Someone who'd be our 2nd top goalscorer, and rating as elite for contested marks for his size, and elite for marks inside 50. Since we rate low for inside 50s, he would need to be able to capitalise on limited opportunities. He'd ideally be very accurate for shots at goal.
Sinclair has tried extremely hard this year but unfortunately this is a bit of an area of weakness for us. I'd argue we miss a strong physical presence in the middle of the ground, like Mumford used to provide. A ruckman to target might be in the elite tier of ruckmen (top half a dozen) for all of these stats: contested possessions, one percenters, ground ball gets, tackles, pressure acts, spoils. Excelling in those areas would really fit into our culture - hard work, no frills, doing the unfancy stuff.