Rather than try and start killing characters off, perhaps we should just return to an old system. There was a time when any wounding or imprisonment would remove characters from their positions.
We changed that for a reason. 90% of the times, the wounded/imprisoned character would have a placeholder keep the seat warm and get it back as soon as he was healed/released. It really didn't give us as much change as you think. There was a LOT of complaining about this placeholder thing, but everyone and their dog still practiced it.