I think my 2 changes have already been touched upon, but the latter derives from the former.
1. Stop considering safety as the most important part of the sport. Life is risk, sport is risk, we balance that risk with entertainment, and there's a trade.
Every law change in the last 30 years has created more issues which have led to more law changes creating new issues... the 20 min red being perfect example in 95 yellows were introduced as a warning, then the sin bin, and here we are 20 years later and there are red reds, 20 minute reds and yellow cards for arbitrary reasons (look at Mauvakas incident this year v Ireland).
The safer world rugby try to make the sport, the less safe it becomes, and the more it promoted physical attributes over technical and skill.
We could take all recent law decisions, the breakdown, making it illegal to touch the lower limbs, waist or head area lol, it's literally a race first to get their wins. The sub numbers, increasing numbers for safety, making props only require 50 mins of fitness, and ballooning to over 150kgs each, the tackle height clamp down hasn't actually clamped down on higher tackles, it's just created a scenario where matches are being thrown for easy red cards, to then have to reduce the impact of a red card lol.
2. A bye product of number 1 really. Sub numbers. Allow for a bench of 5, with 3 to be used as the coach sees fit, and 2 specialist injury replacements. Make all 15 players need to last 80 mins, the likes of Antonio, and other 150kg players would have to slim down or become huge liabilities after 65 mins.