Designated-hitter hook rule
Starting pitchers are throwing fewer and fewer innings as the years go on, which is turning the game into multiple short-spurt outings from starters and relievers who go max effort for one to four innings and then hand the ball to the next guy. I think most baseball fans miss starters going deeper into ballgames, and honestly, it's better that way for both the pace of the game and the entertainment side.
Sure, we all love to see starters who can strikeout the world, and no one here just wants to see guys pitch to contact all day, but there is something to be said about the art of starting and finding a way to maximize results while also giving your team a ton of innings. It's what makes the great ones so special, so how can we get back to that in today's game?
One rule that has been experimented with already in the MiLB is the idea of a designated-hitter hook rule. Basically, a team is able to have a designated hitter in their lineup as long as their starting pitcher meets the requirements needed, or else they lose the DH spot for the rest of the game. Some versions of this rule only require the starting pitcher to get through the fifth inning to maintain the DH, and other versions allow the team to keep their DH until the starter is removed, whether that's the second inning or the eighth inning.
Not only does this rule encourage teams to have their starters go deeper into games, it also adds an element of strategy to the game. If their starter is facing a bases-loaded jam in the fifth inning, the manager will have to weigh the risk of losing their DH to bring in a reliever or see if their starter can get themselves out of the jam. Starters will be encouraged to go deeper into games, knowing that it helps their lineup out tremendously.
If the DH is removed, it also brings back some elements of the game that fans miss, like double-switching or allowing a pitcher to hit or bunch vs. pitch hitting for them. This could bring some strategy back to the late innings of games that I know I miss, adding to the entertainment value late in ball games.