Will be interesting to see how many of these rules remain with AFL only and don't get adopted at local footy level. The play on during a 50m will need tweaking as time on is currently applied when a 50m penalty is given. Technically you can't play on while time on has been called so they'll need to arrange it so that time on begins but if a player plays on, time is automatically restarted.
That is another whole aspect that needs addressing: timekeeping.
I agree that what you say needs to happen. But I'd almost consider doing the same for players lining up for goal and taking their 30 seconds. This would take the need for the umpire to have to decide upon calling play on early if he felt players were taking the piss and wasting time.
Thing is it is not clear cut which free kicks (including free kicks from a mark) you should stop the clock for.
Either way it's worth looking at, and taking the "time on, time off" out of the field umpires' hands gives them one less distraction from their main job.