The new Druid circles have interesting circles that we were able to read and test out. There was circle of the land and circle of the sea. Parts of the 4 main elements. 
Water - circle of the sea

earth - circle of the land 

fire and air - missing. 

should they add fire and/or air?

There already is a wildfire druid. You could homebrew a circle of storms as the air druid. :)

The 1D&D process has been interesting for how the Druid has been presented in parallel with the Cleric. Both, pre-UA6, were Priests with “Channel X” features. To me, this gave a picture of Divine magic being channeled from the Outer Planes and Primal from the Inner. That in turn prompted the idea of different Circles particularly channeling from certain planes. So:

Sea - Water

Wildfire - Fire 

Spores - Shadowfell 

Dreams - Feywild 

Shepherd - Feywild 

Stars - Astral

I tend to think of Land as drawing from the Material Plane, rather than from Elemental Earth. That then leaves perhaps a “Circle of Stone” (because Druids ~ stone circles [albeit not historically…]) and a “Circle of Wind” or even “Circle of the Wing”. 

Not quite sure where Circle of the Moon fits. Material, Feywild or even Ethereal (on the basis of its transitional, liminal nature) could all fit. 

Thanks for informing me about some of the others, but I still think that they should add main circles for Fire and Air. I also forgot to mention that it’s from their play test of DnD 6e. 

Quote from Flyingant125 >>

Thanks for informing me about some of the others, but I still think that they should add main circles for Fire and Air. I also forgot to mention that it’s from their play test of DnD 6e. 

 It’s a nice idea to have the four elemental subclasses, but each class in the playtest has four subclasses and I can’t envisage them dropping Circle of the Moon to accommodate all the elements. We might see them at some later date, if WotC feels that there is a demand for them.