Rolling substitutions would change the game. It would mean bringing on specialists at certain times of the game, very much like American Football. Players would have less playing time, but the intensity of the game would be right up there for 80 minutes, which could be just as damaging to players. It would change the game as we know it.
Not sure what the answer is. Maybe players should only be allowed to be substituted if injured, not sure how that should be policed to prevent faked injuries. What would happen then is there would be a natural drop in intensity as the game progressed,
without fresh legs being introduced against already fatigued players.
I like the idea of a 5m rule from racks and mails, although imagine the collisions off 9 with an extra couple of meters to build up some steam. Maybe the extra room will discourage the use of one off runners with the possibility of more opportunities in the wider channels.