I've been saying most of these for YEARS (like, 5 or so) and I haven't been heard, I know it's a bit late for MLB The Show 21, but there's always the possibility of this stuff being in MLB The Show 22. Anyway here we go!
-3-team trades
-the ability to trade cash
-the ability to trade draft picks
-the ability to trade more than 3 players
-more realistic trades between computer teams
-more realistic FA contracts. I've noticed that in Franchise Mode, teams offer much less money than a real franchise would offer in real life, and the players accept it. Since the real life team and the team in the game have the same amount of cap space and budget, they should be offering more in Franchise Mode.
-spring training non-roster invitee slots
-real spring training stadiums
-more realistic computer logic in spring training. In real life, teams save their starters for a few days or weeks into spring training to try out their younger players. I wish the logic was like that in the game. Also the pitchers should be pitching 2 innings max in the first few weeks of Spring Training, not 4. I just wanna see teams using their minor league guys for a few weeks in the beginning of Spring Training like in real life.
-real minor league stadiums
-real full coaching staff for majors and minors including scouts, managers, bench coaches, 1st and 3rd case coaches, bullpen coaches, hitting and pitching coaches for all levels.
-real draft class
-real umpires if SD Studios can get the rights to them
-the ability to argue with the umpire as your teams manager
-challenges at all bases (I know they've stated that this would take a long time to do but it's on my list for some point in the future)
-more minor league levels
-a schedule for high-A ball and all lower levels SD Studios might add, also more than 15 roster spots on high-A ball. All that team is right now is a storage facility.
-pre-existing injuries
-player progression system like in Madden
-a stat such as "leadership" that would give value to older players who mentor the younger players on that team. As of now, those players are useless if they're not good, but with this stat, they could increase the stats of the young players on their team. This would benefit guys like Pujols and Gardner.
-more realistic player morale factors. For some of the guys on my team, they were already signed to contracts and they were angry that they weren't getting more and that lowered their overall but there was nothing I could do.
-instead of having spots 1, 3, and 4 in the lineup qualify as "star", make 1-5 qualify as "star" because many big hitters hit in the 2 and 5 spots in their teams lineup, like Trout and Judge.
-some extra pre-game, in-game, and post-game content. Like a few words with a player or manager or shots of a post game interview.
-sometimes if a player has a great offensive day, when he comes up to the plate again, they show a replay of his at bats. They should also show great defensive plays that a player has made like a diving catch or robbing a home run or something like that.
-relocation and custom stadium creation (I don't really care about these things but a lot of people do, I'd rather have everything above this one in the game but hey, whatever)
That's all I can think of right now, if I think of more I'll add to this post. If anyone else can think of more things to add to Franchise Mode that I missed please comment.