Well, we could look at red cards for spear and then tip tackles - which didn't improve safety at all, as you can tell by all the spear and tip tackles you see in every match, and all the concussions and other injuries still being caused by these.
Or, we could look at the red cards for tackling a player in the air - which didn't improve safety at all, as you can tell by all the tackles of a player in the air that you see in every match, and all the concussions and other injuries still being caused by these.
Something I have been playing about with, mostly within my own brain, is the idea of ditching both yellow and red cards, and limiting the stoppages at the ground to litigate their use...
I'm thinking that, live, the ref gets to see no more than 4 replays, at full speed only, from at least 2 angles (that actually show the incident), and either shows an orange card, or doesn't.
If there's nothing clear and obvious, then play on; if it looks questionable, then show the card, and the game goes on.
If the card is shown, then the player leaves the pitch and the TMO looks at all the angles (and I mean all of them, not just the ones the broadcaster wants to show them) and decides on the degree of punishment; 10 minute sin-bin, 20 minutes and an enforced replacement, off and stay off. Decision can be put up on the big screen, &/ announced through the stadium speakers (on which, why on earth can Hockey have the ref's mic broadcast through the stadium speakers, but rugby can't manage it)