I have no problem with there being other subclasses that address this. For example, the Circle of the Tides for an aquatic-focused version, perhaps one that has some kind of mechanic to represent the push-and-pull effects thereof. I don't think I would go for a companion creature for that sort of Druid--rather, I'd give it some ability to tinker with its wildshape forms, reflecting the enormous variety of ocean-going life. Jellyfish regeneration and crustacean armor and mollusk intelligence, etc. Although it pains me to say it, technically the Circle of the Land is probably meant to be the "mountain"/earth druid, in the sense that it's linked to terrain and physical structure. I'd personally prefer to make a subclass that gets superior defenses and such, perhaps even the ability to give yourself "earthen armor" as a substitute for metal armor. Wind is trickier; back in 4e you had the Eagle or "Watcher" Shaman who was good at supporting ranged abilities and physical movement around the battlefield, so perhaps features that hook into that would be functional--not sure, would want to dwell on it more.
This, incidentally, is part of why I balked at someone else's suggestion of "improving" the flavor of Wildfire by making it "choose your element": that actually makes the subclass even blander. If you actually make them different, e.g. "well you have the Earth package, which gives its set of features at level 2, 6, 10, and 14, and the Water package, which gives genuinely different features at those levels," then you've already made four different subclasses and you're just pretending they're one subclass. If you don't do that, then all you're doing is making it "Insert Element Here," which must inherently be more bland than element-specific stuff.
