Free substitution means that a player may leave the game and return later. Pretty basic. Football and basketball have it. Why not baseball? Just because they didn’t think of it a million years ago? Or in 1950 when there were the last big modifications of the rules? …
To make this work in baseball simply maintain the batting order. If Derek Jeter starts the game batting second he may only bat in that position. I am guessing that the other objection would be that changing pitchers constantly would slow down the game, as if anyone would notice. Change on the fly like the other sports. As long as the move is done within the flow of the game there should be no problem. Of course, they cannot continue the pitcher changing ceremony that is unique in team sports for its disruption and delay and which should be eliminated anyway.
9. Players allowed to warm up when entering the game. The backup quarterback does not get to take unlimited time to replace an injured starter. MLB has an entire replacement ritual for replacing a pitcher, including a coach waddling out to the mound to initiate the change. Can you see Tom Coughlin taking his sweet time walking to the middle of the field to speak to Eli Manning and decide whether he wants to replace Eli? Then signalling for the backup who must warm up while the other 21 players wait. Yeah, right!
To further clarify: players may only substitute for players in the same position in the batting order. For instance, once a player substitutes for a player who was batting third he may only substitute for a player batting third. The original player batting third may only return for another player batting third. It does not matter whether the substitution takes place at bat or in the field. Only one of the players batting third may be in the game at one time. It’s really pretty simple.
In the field they may play anywhere just like now. With only about 12-13 position players I don’t think that managers will stack more than two in any particular batting order position. It could allow for defensive specialists at maybe three positions. And pinch running specialists: Herb Washington is only 60. In 1974 Washington was Oakland’s “Designated Runner”.
With a Designated Hitter (DH) rule, the pitchers would simply substitute among themselves and there could be multiple DH players. If the pitcher bats, then the general rule would apply. A pinch hitter for a pitcher may pinch hit again for any pitcher. It would essentially emulate the DH.
More players would play and there would be more specialists, which would improve the quality of the game. Like in football and basketball.