A file found by SirGawain8 shows that the creators are working on new areas. The areas shown on it are Bug Garden, the moon, a farm/field, a swamp, and a jungle/rain forest. The moon might have been the Ready Jet Go! event room. And another was a waterpark. (Waterpark is not on the photo).
It was later confirmed that there will be a desert area and a secret cave area sometime in the future, but some people wonder what's going to happen to the other new areas like the farm or the moon. Only the desert, jungle, and secret cave have been confirmed via the Kart Kingdom Blog so far. The desert appears to be the area that will come first as seen with The Run-Around Gang, who are from the Desert Area, and some files about racing that include desert terrain.