Spurred by some of the Planar Renovation discussions about the inner planes, I had an idea about demi-elemental planes. These would be planes resembling the elemental planes, but of a related element (and much smaller, more focused). These demiplanes might exist near their corresponding elemental plane. Some examples:
chlorine (near Air)
acid (near water)
blood (near water)
These planes might form spontaneously, or be absorbed by their parent plane at some point. The quasi and para-elemental planes might have formed as such, then become fixed between the two appropriate planes. This could suggest that other para-elemental planes existed in the past, and were replaced (kind of like gate-towns?)
This could also explain the alternate para-elements of the Dark Sun setting (replacing Ice with Rain, Smoke with Sun, and Ooze with Silt). We could also draw from the Ravenloft elementals (Blood, Pyre, Grave, and Mists) as an interesting set of demiplanes.
And we could create even more strange ones. I remember an old dragon magazine joke about a beer elemental 
There's some merit in exploring the concept. Personally, I'm fond of the idea of the para- and quasielemental planes shifting and changing around via some mechanism. I think my biggest concern would be diluting the elemental concepts embodied by the various planes, but as long as the plane itself is limited in scope, it probably wouldn't be too big of a problem. Especially if it ends up working out that the demiplane gets absorbed back into its parent. Actually...that might be really appropriate for how the Inner Planes work. Kind of a survival of the fittest amongst the Inner Planes, where only the planes that have the strongest internal reality to resist being absorbed into the planes on its borders.
Hmm...