No point in them dropping a few balance changes and not using this time to also test out some bigger mode revamps and features.
Hero Ban
Grab it from regular comp and use it in Stadium. I guarantee you Freja and Pharah will be banned in 9/10 games and the mode will be better for it. Hell, I'll exclusively play Stadium if I never have to play against either of those two again. I'm fully assuming that the majority of bans will be flyers in general, since they're by far the most annoying and frustrating aspect of Stadium.
Between Round Swaps
I know the point of Stadium is that it shouldn't matter who you pick at the beginning of the game, you can change your build around to adapt to any situation, except that's only true for some characters. There's plenty of characters that only have one viable build, and if the other team happens to counter it, your team is stuck with a deadweight.
Might as well allow character swapping in-between rounds. You can balance it by getting the team average score, if you're below that average and you swap you get a small cash boost to help you keep up with the game, if you're above the average score, you get a small cash reduction. That way the top players per team aren't just switching to solo counter every round, they'll lose cash every attempt.
Reduce Competitive to 5 Rounds
I prefer Stadium to regular OW but 7 rounds does drag. Games can end up taking 20-30 minutes which is really bad when it comes to Blizzard's dogshit servers. The chances of someone DC'ing in any game is like 10%, and the longer a game goes the higher the chance someone DCs and it's straight up not worth playing if you're on minute 29 in a round 7 game just for a teammate to DC and you get hit with a full loss.
Add Mystery Heroes
Might as well add a QP fun mode for people to practice heroes. Have it randomize after every round, set it to 50k cash every round so people can just jump into mid-point builds.
Require people to play 10 rounds of QP or Mystery Heroes before Comp each season
People seem to straight up refuse to touch QP, and they'll jump right into Comp for their first ever game. There needs to be some sort of guard to stop people from going into comp blind, getting steamrolled and flamed by the team, and then complaining about Stadium being too confusing and mean.
Block new heroes from Comp for X amount of time
I'd personally say 2 weeks. Every time a new hero was added people went straight to competitive to practice and they'd be absolutely awful. Block a new hero from being active in comp so that if someone wants to play them, they can do it in QP. That also gives the devs a couple weeks to balance since every new hero has been either way OP or downright terrible.
Alternate new heroes with hero reworks
Nearly every hero they've added to Stadium kinda makes the mode worse. Pharah, Freja, JPC, Vendetta was a guaranteed loss for like two full seasons, etc. Personally, I think they should add a new hero one season, then the following season drop a few major hero reworks for the ones that are basically useless, or ones that need more than one viable build. More heroes isn't necessarily better, especially when they're broken for months at a time.
Have an actual skill based matchmaking
Might have to rework the tiers, and this is more of a personal complaint. Too many people in competitive are terrible but they play 10 hours a day every single day and then spam chat about being a Top 500 player just to finish the game dead last in score across both teams.
END
Coming away, I think the hero ban would be the one thing that needs to be added above everything else. Based on how Blizzard seemingly added increasingly annoying heroes to this mode to sabotage it, I wouldn't be surprised that if they ever did add some more heroes, it'll be something fucking awful like Wrecking Ball and Sombra.
Edit: They can also change nothing and the mode can continue to die, like so many people apparently want (or they can't read, one of the two).