That's what I'm thinking too, if Gamefreak was to create another type it'd be something nobody or very few people fans would have thought of before hand and that might end up quite complex.
I mean if you look around, lots of people still don't quite get the fairy type and try to shoehorn it into either "the mythology type" or "the magic type" when no, it's the Fairy type and it might contain elements of folklore and magic, but neither does it cover all of folklore/magic, nor do those two aspects cover everything the Fairy type represents.
From the commonly suggested types I guess the Sound type seems the most like it could be a thing. Sound is a phenomenon that reacts very differently to various substances and aspects of nature. Like it could be weak against Grass (sound boosts plant growth) and Ice (noises can cause avalanches) and strong against fighting and psychic maybe? (use sound to break their concentration)
But even with sound there's quite a few problems (too many song and noise based attacks that are already other types etc.)
The one type I could kinda see working is Mutation type. It be explained as new and possibly as a result of humans' tech and interactions with Pokemon.
And....what would that be? How would it translate into attacks and Pokemon designs that differ from other types? Especially in a way that woulnd't be "sooper edgy" and have Pokemon run around with cancerous tumors, chemical scars or deformed extremities..
Pollution and mutation because of environmental degradation is kinda covered by the Poison type with things like Grimer (thought to be fair Grimer could be retyped to Poison/Mutation if they were to introduce a type like that) and it seems the dark type also covers some of the adaption to environmental degradation, as seen in Alolan Ratata and Grimer.
But, like, what would a "Mutation" type Eeveelution look like?
Theoretically, a type representing environmental degradation would be a natural foil for the Fairy type if it ever becomes OP, but in practice it might be difficult to keep it in line with Pokemon's overall optimistic tone and cute designs.