For the last 3 gens we got gimmicks that were only related to some pokemons but i really want to see something more broad on the board and it struck me that the field, aside from weather, isnt much of a factor in the games. My take would be to make it like Chrono Cross and make moves change the type of terrain the battle is taking place and for it to have some effect on pokemons. The lore behind would be that the new region would be a special kind of volcanic island where the soil is infused with energy and mon types affect the terrain and vice versa. The main gimmick would be
>Making the field the type of your mom for some bonus and to criple the opponent
>The type of field affects both sides equally
>Takes 3 moves to fully change
>Weather moves apply 1 instance of change to the field the same as its typing
>Gaiastone is a new iten that lets you concentrate a field effect(that has been applied that match) to apply it for one turn (Once per battle)
>New pokemons in this region have abilities to change more than 1 stance per move or even negate the field change
>New moves that behave different on each kind of field

Now what would be some cool effects for each type? Was thinking Flying would have tailwind for one turn or something