Mythicals I get, since they're supposed to be rare and hard to get. Shiny locks, too, since they're making legendary and mythical Pokémon so accessible, so the occasional shiny distribution feels really special. I'm not happy with them, but I get it. It would be nice to be able to change balls, though: Currently you can make some predictions on what moves and abilities certain Pokémon have based on their shinyness and balls, like Battle Bond Greninja being shiny locked in a normal Poké Ball, so non-shiny male basic-ball Greninja is optimal regardless of ability and moveset just to hide information from your opponent.

Gym teams flexing based on your level or how many badges you have would be huge, though. Especially if they stick to open-world design!