So will both black and red cards be used?
If it's gonna be just yellow and black then a coach might start a couple of players with the aim of them injuring a high value opponent and then being subbed.
There are only really two red card offenses that we see the majority of the time. Shoulder charges and playing players in the air.
Shoulder charges and armless tackles should certainly be red cards. The only example I can remember of where a shoulder charge was harshly punished was the SBW one in the Lions series. The Lions player seemed to unexpectedly step toward SBW's half of a double tackle and SBW dropped his shoulder half in self defense and half in brain fart. For the most part shoulder charges are clearly intentional.
The most dangerous thing that we see is players being taken out in the air. And strangely it doesn't look nearly intentional most of the time. Regardless it is dangerous as hell. There is no way that causing someone to land on their neck should not be a red card. And the team guilty of that infringement should be penalized severely. As long as players are allowed to jump to catch balls we will never see the end to such instances.
For me there are two options. Either penalize any player who jumps to take a kick. Or make so that only either defender or attacker is allowed to jump and any contact made with him before he lands, even if he lands safely should be penalized.
Everyone makes such a big deal about high balls and contests in the air being such a huge part of the game. I just don't see it.
In fact, if neither player was allowed to jump we'd see a lot more interesting situations arising. 15s would probably need to be taller and there'd be new techniques necessary for taking the ball above ones head without jumping.
The vast majority of games I've seen ruined have been because of those two fouls. There is no need to change the entire carding system. Just sort out the main culprits and we'll be most of the way to finishing games with 15 in each side.
Interestingly I can't remember the last time I saw a red card for anything else.