The biggest problem the game has today is the pace it has been deliberately slowed to. This has led to the rise of the rugby behemoths, where rugby players in all but one position must be at least 1.8 metres tall and be somewhere between 100 and 130 kgs of muscle. Park rugby might be played by people of normal dimensions but professional rugby is very different. Games are nominally of 80 minutes duration, but most only consist of 30-35 minutes of actual play. The rest of the time is consumed by stoppages - contrived injury stoppages or patterns of play set-up that take a ridiculous amount of time. Preparation for penalty kicks, preparation for lineouts, setting of scrums, stoppages for minor injuries (real or imagined) all serve to use up valuable playing time.
The solution to that I believe is very simple. You have make an 80 minute game contain at least 60 minutes of play. Then fatigue comes into play and skilled players and fit players can take advantage of that. You need to change some rules to make play more continuous (probably not the ones you are thinking of):
So I propose the following:
1. All penalties must be taken and be complete within 30 seconds of the referee making the mark. The TMO does the calc and immediately notifies the ref that time has expired. Failure to complete in time is a free kick to the other team.
2. Lineouts must be taken as soon as the referee arrives at the spot in line with the AR. Failure to take the throw immediately is a free kick to the defending team. Mucking around by either team is a full arm penalty to the other.
3. Unless the injury is catastrophic - head or neck injury or broken bone - play will continue while the player is treated. An immediate substitution is allowed if the player injured is critical to play ie a front rower. That player has 20 seconds to get to his position while the other player is being attended to. Officials will not be allowed to interfere with this process to slow it down unless a team is clearly cheating. The substitution can be reversed within five minutes without penalty if the injured player recovers. Otherwise it is a normal substitution.
4. As soon as the ball is available for the halfback, the referee shall call "balls out" and defenders are free to advance beyond the offside line. The current caterpillar ruck is banned.
The attrition rate for the first few matches would be interesting but it would soon settle down into 60 minutes actually played in every game and the spectators would rejoice.