You know, I've been thinking about Mega Evolution as of late. I think it's the best of the super mode mechanics we've had so far, and if there was any of them I'd keep, it's Megas. It's the only one with substance behind it as it gives Pokemon new stat spreads, new abilities, and even new typings and it added a lot to many Pokemon and was fun in competitive battles.
But that's beside the point: one of the criticisms about its execution was how much it breaks in-game. You can liberally use it and the mons effectively trivialize the game once you Mega Evolve them.
Then I've been thinking...but I think there's one easy way they could fix Mega Evolution if they ever put it back in to control how OP it is in-game. Heck, it could've been done back in Gen 6 itself.
And the biggest way to regulate Mega Evolution imo...would be to connect it to Pokemon Amie.
It was right there from the start. Mega Evolutions are a cool mechanic, but I also really love Pokemon Amie. It's such a cute feature, and it's adorable to play with your Pokemon and forge a bond with them. Feed them, interact with them, vice versa. And in doing so, the game also has mechanics of affection and enjoyment, especially the former which emphasizes how much the Pokemon loves you. It helps really push the notion of making your Pokemon your friends, so much so that through little things like this you forge a stronger bond with the mon, and it shows in battle where they can do stuff like crit or heal poison or endure a hit because they love you. It's so neat.
And that would be the perfect way to regulate Mega Evolution as a mechanic. How? Because one of the key thins about Mega Evolution in lore is that they make it very clear that it can only be achieved through a strong bond between Trainer and Pokemon. That is one of the big things both the games and the anime have tried to push, and the SM dex entries even talk about how painful Mega Evolution can be for many mons. I believe the Gen 7 dex entries are meant to emphasize what would happen to a Pokemon if it Mega Evolves but lacks a strong bond with a Trainer, and so much so that if they don't, then they're in pain and will go wild, but with a strong bond they are in total control of that power. The XY anime even emphasized this with how Korrina's Lucario went wild and was very evidently in pain when Mega Evolved at first.
So I think a good way to regulate Mega Evolution's effectiveness in-game would be to really tie that into Pokemon Amie as a requirement for it to be fully effective in battle. To tie in to what would happen if a Pokemon Mega Evolves in the absence of a strong bond, make it so that if the mon doesn't have enough affection points from Amie, then it can Mega Evolve, but it would go berserk basically. Completely disobeying you and using moves at random, without you being able to command it. Not only using moves at random, but have a high chance to frequently hurt itself until it faints. Many SM/USUM dex entries even talk about how some Mega Evolved Pokemon can't even hear the commands of their own Trainer, or are in so much pain they act wildly. Make that happen in-game, basically! And maybe as Affection points go up through Amie, that happens less and less. And once you've achieved maximum affection with the mon, then you've mastered the ability to use Mega Evolution with the mon. If you have max Affection with the mon, then the mon even when Mega Evolved will then listen to you without fail, and you can fully utilize the mechanic.
Attaching Mega Evolution to Amie and its related derivatives would be an excellent way to not only regulate its effectiveness in-game, but it's incredibly beneficial for both features. It adds a layer of substance to the Mega Evolution mechanic, ties it into its canonical lore effectively, and also further incentivizes the use of the Pokemon Amie feature by encouraging players to use it as a means to master Mega Evolution and achieve the bond with the Pokemon and put in the work and effort they need to fully effectively utilize Mega Evolution. Sure, you have the Key Stone now, but that doesn't mean you've mastered the bond with your Pokemon just yet. So use Amie and work on developing the bond with your Pokemon and in doing so, your ability to control the mon when Mega Evolved strengthens with it.
I think this would be an excellent way to improve the Mega Evolution mechanic. Of course, I don't think the fools at Game Freak even realize this is a possible thing they could do, but if there's one way to improve Mega Evolution, this is almost certainly one that would work imo.