>>43256449- I would name the difficulty levels Rookie, Adventurer, Champion and Master, the latter being unlockable once you beat the game in Ace mode.
>RookieEasier than current Pokémon. Plentiful items, easy as pie AI and NPCs healing your Pokemon everywhere.
>AdventurerThe same Pokémon experience as B/W. A very forgiving game that gives you many second chances and a couple NPCs that heals your party. AI isn't too braindead.
>AceNo NPCs heal your party, items are more expensive and Max Revives are a rarity (only one in the entire game). Gym leaders have 5 Pokémon and all of them are 5 levels above you, no matter how much you grind. They switch their Pokémon if things go badly and will use many Full Restores.
>MasterUnfair game experience. Your bag size is limited to a very small size, you can only save your game inside Pokémon centers, which are located only in big cities. Gym leaders will switch their roster to Pokémon that are strong against yours and will use Max Revives when in trouble, so you don't really know which Pokémon will gym leaders use against you before you battle them.
That may be it without changing significatively the game engine.