The one rule-change I've always thought would be a good idea, that I don't think is even a notion at this point, is a "# of pitcher minimum" in a nine inning game.
Speeding up the game and marketing the game to millennials is one thing. But the starting pitcher is dying. An "opener" or a "bullpen" game was a cool idea when it was first implemented. But it's killing the starting pitcher, which has always been a staple of the sport.
My proposal: Teams are only allowed to use 4 pitchers (or 5, I'm not set on a specific #) in a nine-inning game. You pull your pitcher after 5 innings, let alone 2 or 3 there, and there are consequences. Make it an advantage for a team when its starting pitcher goes 8 and 2/3 innings. Then you can mix and match to your heart's content to get the last four outs. But if your pitcher can only eke out 5 innings then, well, as opposed to having a cadre of relievers you can deploy over the next 4 innings, then you've got a tougher road ahead.