>difficulty options
>you can only use the same amount of Pokémon as your opponent during important battles
>gyms aren't monotype, but instead follow a certain theme (ex: Music gets stuff like Toxtricity, Chatot, Exploud, etc)
>every npc Pokémon has an item
>all battles are double battles. If you want vgc to be a thing, GF, make the main game like this, you inept code monkeys.
>restricted amount of items usable in battle
>level gain halts or slows down significantly depending on the gym badges you have
>battles are set, because switch makes absolutely no sense
>old Exp share is back
A funny little mechanic I would implement that doesn't necessarily affect the difficulty would be trainer classes. These can affect the way you gain exp and certain items, or even what Pokémon become available to you. It can also lead to a Join Avenue like feature where you can trade/sell items that other people have more difficulty obtaining.