It always could be, but a nature based damage dealer would have a lot of overlap with druid's aesthetically. Even more than demon hunters had with warlocks and rogues. Arguably more than dark rangers would have with hunters.
There was a thread a while back suggesting Dark Rangers and Necromancers be specs within a class which I liked, and I guess some sort of pestilence/undead "druid" could be a third spec for that class. Calling on swarms of insects, corrupted roots, thorny vines, etc.
Probably better off altering druid to fit the role though.
Like this, which I would absolutely love.