I agree about kicking the ball dead. I would suggest an even simpler solution - rather than on the 20, let the tap be taken from the 30. That's a pretty decent attacking platform.

Unless the attacking team is within the 20, otherwise it would eliminate a grubber on the 5th

Ball goes dead in goal when kicked within the 20 then 20m restart.
Ball goes dead in goal when kicked between the 20 and half way then scrum defence from point where ball kicked.
Ball goes dead in goal from attacking teams own half, or ball goes dead in goal on the full (from anywhere) then penalty defence from the point where ball kicked.

Having thought about this surely a kick that goes dead is punishment for inept fullback play. Saying that such a kick should be punished takes away any responsibility on the defending team to field the ball.