PROPOSED RULE:
A 20-second pitch clock
This could drastically change the way we watch baseball, and the way pitchers/catchers have to make decisions. But anything that makes things happen faster, without sacrificing the integrity of the game, is usually better for the sport (see NHL icing/twoline pass/reduced neutral zone/ overtime changes, or the NFL reducing wide receiver contact in the first three yards).
While it might take a slight adjustment for the pitchers/catchers, most of them have been in favor of a clock, with hitters often the ones who seem to have an issue getting into and staying in the box.
For context, the fastest pitching team in the majors last year was the Mets, averaging 22.7 seconds between pitches, according to FanGraphs. The Red Sox were the sixthslowest at 25.2 seconds. There will undoubtedly be an adjustment period, but it seems to have worked well in the minors.
Commissioner Rob Manfred has the power to implement this rule without support from the players union, so this one seems inevitable, perhaps as soon as this season.