Make scrums contested again.
The problem was with the number of scrum infringements, and referees penalising teams and thereby influencing the match result. The introduction of the differential penalty helped but there were still too many penalties.
One idea was to have the ref feed the scrum. But this didn't resolve the dozen or more other scrum infringements on offer.
My former forummer self once proposed an idea where the referee would force a play-the-ball rather than give a penalty. That is, the team breaching scrum rules would essentially lose the right to contest the scrum. That should (in theory) decrease the number of scrum infringements.
So yeah, I want to see contested scrums. For that matter, I also want to see striking in the play-the-ball brought back.
I know the limited tackle rules ultimately brought about these changes, and that has been a great innovation. But imo we lost one of the dynamics of the game when contesting the ball was all but abolished during the 1980s and 90s.