I think the rule has more to do with preserving pitching staffs than shortening games. So, maybe your suggestion could work.
Manfred said the other day, that they are going to try to figure out a way to get staring pitchers to go back to pitching more innings because that's what fans want. I don't think that can be done with a gimmick. It would require some kind of substantial change. Chas has suggested many times that deadening the ball would fix many of the games problems - too many strikeouts, not enough balls in play, starters getting burnt out. The problem is they won't give up the long ball.