The following rules could use some changes: DH, Strike Zone/Robo-ump, Shifts, Posey Rule.
DH: Presently a DH cannot be moved to a fielding position w/o the team losing the DH. Catchers are the players most impacted by this rule. If one catcher is DH and the other catcher is hurt, then the team loses their DH if they move their DH to catcher. This prevents teams from using a catcher as the DH since teams usually have only 2 catchers on the roster. I suggest removing this rule. They made a special exception for 2-way players (the Ohtani rule). They should just drop this restriction all together.
Strike Zone/Robo-Ump: The move to the robo-ump is coming. In the minors they are trying an approach where the robo-ump is not used on every pitch. Rather if a batter questions a pitch, he can ask the umpire to check the robo-ump call. This is similar to the check swing appeal rule. The team is limited to how many appeals they have. I like this approach better than robo-ump on every pitch.
Shifts: I'd still let teams shift but it would be required that 4 players must stay on the infield dirt until the ball is pitched. I have seen more grounders to the right side be gobbled up by one the fielders playing in short right field. Batters would still be incentivized to go the other way by allowing teams to shift. Left handed hitter would especially benefit by this change.
Posey Rule: It is still not clear to me what the present rule defines as blocking the plate. I'd make it clearer by requiring the catcher to stay in fair territory at all times; even if he is trying to catch the ball or has the ball in his possession. The runner would then be required to stay in foul territory until he reaches the plate. This way each player has their spot on the field they are required to use.