In my opinion, Druids tend to be at one with nature and revere it. Monstrosities are at odds with the natural order of things and are a bastardization of fauna*. I would prefer that Druids only be allowed to transform into Beasts, no matter what Doric did in the movie (although, a specific subclass, like Moon, for other wild shape forms would be acceptable).
*according to the Monster Manual (p. 7), "Monstrosities are monsters in the strictest sense - frightening creatures that are not ordinary, nor truly natural, and almost never benign."