What if Pokémon's Type interactions were also affected by species? I DON'T support this because I think it's just too much to expect players to memorize them, even if very few Pokémon had this. But I want play with the idea in my head.

Basically, a very small minority of Pokémon species would have a Type interaction that their typing alone doesn't cover, where it makes sense. So, since it is literally stated that Magcargo are so hot that it vaporizes water on contact, Magcargo would now innately resist Water instead of being weak to it.

Another one is that Durant is specifically stated to have evolved metal armor in order to STOP Heatmor eating them. It should resist Fire Type attacks, not via an ability or anything else but just innately. It's said that punches and kicks slide right off of G00dra's slippery body, so it could resist Fighting. Parasect is a brainless zombie, so Psychic attacks could fail if used against it.

Gumshoos should resist Dark, since it basically fights crime and has adapted to prey on Alolan Rattata specifically. Seviper could resist Normal while Zangoose resist Poison. It should be a tiny minority of Pokémon that have these in order to keep it simple, but there are lots of ideas here.