Two things that hugely slow down games: the time between half innings, when teams leave the field to come to bat, and pitching changes.
Most often, a team makes three quick outs, nothing across, then must get their gloves and get out there. The change can take longer than the at bats. What is needed are fewer inning changes, and more time actually playing the game.
So here is an idea, a radical one, at that: games are reduced to 7 innings, but with 4 outs per half inning. The total outs will be one more (28 v. 27) in a full game, and allowing 4 outs will no doubt stimulate and increase scoring.
Pitching changes are obvious time wasters. MLB Could limit pitching changes to 1 per half inning. A reliever starts the inning, gets 2 or 3 outs, then another reliever comes in. No more changing pitchers for each batter for lefty-righty match ups. Even eliminate managers coming to the mound to make the change. Signal to the ump, tell your pitcher to take a seat, wave your left or right arm, new pitcher sprints in from the pen.
both of these radical ideas would cut out a lot of down time in the game that is merely filled by commercials.