Years ago, MLS has a separate reserve/B-squad league play that really didn’t work well. I don’t remember the logistics of the blunderous effort, but I think part of it was the smaller size of rosters, and coupled with injuries, there weren’t enough players to fill out the B-squads so teams either didn’t play matches or they called up guest players from local colleges or D3 teams.
But now that squads carry ~28-30 players, even with a few injuries, there should be enough to play 14 in the marquee MLS match, and then those subs could be available (if playing less than 45min) to supplement the rest of the B-squad to give the staff options.
The difficult part of the logistics is when to play the match since most teams want to fly home right after they play, so the B-squad would either be on their own, or the team delays departure; playing beforehand is out as it would tie the coaches hands on 1st team selection if there’s a late scratch. Maybe get the game rolling 10min after the final whistle and it wouldn’t end that much after the main squad finishes in the locker room and the post-game presser? Dunno, but it’d be a great way to get guys game minutes each week without having to be shipped out to an affiliate USL team, and the style/tactics could be kept the same by having the homogeneous practices instead of guys training with the USL team.