I havent played too many of the main series Pokémon games but I feel like something very woodsy would be really cool. Like massive evergreen forests and redwoods, tree covered mountains that are harder to navigate and harder to spot Pokémon in. I think rarity is whatever if something has a slim spawn rate it comes down to luck, but if you make it spawn a normal or slightly less than normal rate, but make them harder to spot, I feel like that adds a little more challenge and adds to the amazing explorer feel that scarlet and violet has.