Remove Megas. Either return to the pre-mega universe or some energy or radiation has flooded the world in the time between XY and SM that renders Pokemon unable to Mega Evolve. It will absolutely never happen, but it'd be nice.
And specifically for my favorite Pokemon, my Gen 7 wishlist for Mismagius:
Hidden Ability is Ghost-type Pixilate (Spookify!)
Hoping they do another round of +10 buffing to stats, and hoping Mismagius gets a +10 Speed boost.
Added Fairy-type and access to Moonblast. If not adding Fairy to more existing Pokemon, at least give access to a new Fairy-type move with slightly lower power than Moonblast with a decent secondary effect. Something that could force switches or otherwise screw with an opponent. Something more powerful than Dazzling Gleam. I could see a magical Fairy ability that pacifies the opponent, doing damage and having a flinch chance. A sound-based one would be thematically perfect. I don't care who you are, you're going to flinch if a Mismagius screams right next to you. Fey Cackle? Fey Screech?
Access to Mystical Fire, and all other remaining "Magic" moves that make sense. Access to Roar because it's a banshee. I don't think anyone will run Roar on Mismagius because of the general frailty. Eviolite Misdreavus may appreciate it, though. Misdreavus also gets Sonicboom as an egg move because it's a banshee.
Spookified-Hyper Voice becomes most powerful Ghost-type move in existence. Other potential attacks in a moveset have secondary effects that can annoy and force switches. Mismagius screws with people. Its existing status and support movepool is wide enough for this already, I think. Otherwise, run Nasty Plot and blow holes in teams with your fast and boosted banshee wails. It's still pretty frail, but the raw power of a Spookified-Hyper Voice combined with Nasty Plot means you could sacrifice some Special Attack EVs to supplement your bulk if needed, and it gives pure annoyer/support Mismagius respectable attacking power without a Sp. Atk investment (like what Pixilate Hyper Voice did for bulky support Sylveon).
Also, ditch that horrid mustard shiny and give us a gray-and-white or a black-and-gray Mismagius. With deep purple eyes and gemstones.
...Or, don't make Spookify and give Mismagius Boomburst? I'd rather have boosted Ghost-type Hyper Voice than unmodified Normal-type Boomburst. You can't give Mismagius access to hypothetical Spookify and Boomburst at the same time because that'd be insane. It's a shame that Boomburst is thematically perfect though.
Mismagius also loses Ground-type immunity with Spookify, so that's another vulnerability for it.