From what I understand the current plans are to make it so that the terrains set by the terrain abilities (as well as moves that set them, like Mist) do not actually replace the field. Instead they will be on top of the fields, so that you get the effects of both without replacing the fields. Obviously I am not a dev but that is what has been mentioned on the discord.
(Not a dev but) Considering that there are a few characters from the official games (I won't spoil because I get the feeling you haven't reached that area yet), I feel like I can give the answer of "yes".
Most likely it will only be a limited number of official characters for a smattering of reasons.
Yamask and therefore Cofagrigus is available in Goldenleaf. Galar Yamask and it’s evo will be added somewhere, and it’s evo method will be changed in a way that makes sense. It is the same case as the Alolan forms evolving on Terajuma or alolan marowak evolving in the goldenleaf area.
The old design still has rainbow hair, except it is just pastel. As for the skin color it is now closer to crawlis, which makes sense (also it’s still relatively the same, the shading on the art is different).