It just occurred to me, looking over some of the posts (including my initial one)...that several of the circles we are talking about could be a single subclass...with the addition of a secondary warlock-pact-like choice point built into IT.
We have the Circle of the Land: druids who are tied to a particular terrain choice/region of origin/speciality. And the Circle of the Moon that is for druids who are predominantly shapeshifters. The druid BASE class is still a full caster. Spells and magic are still the druid's primary power in either case.
So you COULD simply add:
Circle of the Touch [or whatever, working title]...actually, Circle of the SHAMAN works for any of these, as well:
Druids who are predominantly dependent on a particular aspect or facet of the natural world. These are druids who -through years of concentrated meditation, specialized training, or some innate ability- connect to, have an affinity for, and exert power over a specific PIECE of the natural world moreso than any others.
Aspect of Nature
At second level, when you choose this archetype, you must select one of the following areas in which to focus your meditations, knowledge, and magics. Once chosen this, Aspect of Nature can not be changed:
Aspect of the Elements: You have an affinity for and command over one or more of the following: air, earth, fire, water, or via combinations of them, weather.
Aspect of the Spirit: Spirit shaman, per normal, as discussed.
Aspect of the Beast: Animal communicator/controller/summoner, as discussed.
Aspect of the Green: Plant communicator/controller/summoner, as discussed.
Come to think of it...this kind of thing would work REALLY well for a break down of Witch archetypes as well. Though I've long considered the Witch -as a class- more aptly constructed as an offshoot of Druids than Wizards.
Here, in this single subclass of "controller/summoner/dependent and focused on some piece of the natural world," you could make your Elsa/Iceman, Storm from the X-Men, your Last Avatar Air/Earth/Water/Fire "Benders," Poison Ivy/Swamp Thing/Hedge mages [maybe even Groot?], your Game of Thrones' Worgs, Beastmasters [of film], even Tarzan!
It certainly has the scan and scope to generate as an independent class with its own subclasses...but since they are all, already, Druid offshoots or [at least] Nature-themed archetypes, why bother? The Druid already exists. All of these could easily be types of druid...within a single subclass.