I miss the amount of single player activities you could do in the postgame. Since Gen 6, shiny hunting, competitive battling, and a short postgame quest are all you can do other than completing the Pokédex. In older games, battle facilities had more going for them and there were large chunks of the region you could explore. Now I feel like you have to be a shiny hunter or a competitive battler to get anything out of the postgame.
I honestly forgot about this and oh my gosh I miss it so much now! I honestly miss just the art style. Things seemed alot more lively and less generic anime people. Some cutscenes are pretty impressive, and this could just be my nostalgia talking, but I 100% believe the old style is better than the new style.
Triple and Rotation battles. They were a neat little gimmick in Gen 5 (B/W/B2/W2, I think) that I was starting to fully understand and use to my benefit when they were scrapped.
I miss old sprites and cries- my beautiful purple boi Drapion is all washed out, and not to mention his horribly washed-out shiny. One of my favorite shinies now looks horrible. And cries? Some cries were distorted to “sound more realistic” but for example, Charizard and Kyogre’s cries in the 3D games sound so bad.
Seasons and Pokémon variations, like deerling. Though of course in places Alola it'd be relatively the same all the time based on the location, but I think Kalos could have just had seasons.