What if they did something like expanding the roster to 27 players but that extra player can only be used as your extra innings pitcher?
He can’t pitch at any other point in any game and you cannot swap that person out with anyone on the active roster. To change to a different pitcher in this spot you have to actually use options or DFA them. They only exist to pitch extras. If they have to come out due to injury, then you forfeit.
This way you aren’t frying your bullpen by going into extras and the longer they pitch in extras the more likely they run out of gas and start giving up hard contact. Might be an interesting strategy on how teams manage that roster spot and who they put there. Do you sign a stud and have him just ride the bench until you play extras, or do you put a gas can out there and use your best pitchers in regulation innings.
Likewise, once it goes to extras, you can’t pinch hit anymore unless there’s an actual injury. Lineups and pitching staff is locked after the 3rd out of the bottom of the ninth.