Well, there are some things I would change.
1. No more designated hitters allowed in the AL.
2. Allow non-wood bats to be used.
3. No more closers allowed. It's kind of crazy but this is how it would work. You would have to keep in your lead pitcher for the entire game. You can pull him if he gets injured, but he has to be replaced by someone already on the field. And then, the guy who replaces the pitcher would be replaced by someone from the bench. The pitcher cannot return to the field. The guy who replaces the pitcher has to pitch the rest of the game. If he gets injured, then he has to leave the field and not return and he would have to be replaced by someone else on the field. This way, if a manager wanted a back-up pitcher to get into the pitching rotation, he would have to replace an outfielder or infielder with a pitcher, and then have him throw at least one pitch. And then replace him with your back-up pitcher. But to prevent constant pitching changes as is done now, there would be a rule where anyone who is relieved of pitching duties cannot bat anymore either.
The main reason for this change is to increase the number of hits between the 7th and 9th innings. When you have fresh pitchers put into the game, they can shut down the opponent for the rest of the game. But, if you take that away, a team can really come back in the end because the pitcher is tired. A lead isn't guaranteed anymore.
In the past, I've seen games where a team's bullpen is emptied out and the last guy on the mound is getting killed. The only way to end the inning is for the infielders and outfielders to tag runners out.