Or, How to set up Elemental Demiplanes that are Simpler than 2E but More Varied than 3E. 
So I've been thinking, recently, on the nature of the elemental paraplanes and quasiplanes. IMHO, they were a fairly varied lot - some were fun and unique places to visit (such as Ice and Mineral), some were kinda redundant when set against each other (Ash and Dust come to mind here), some seemed difficult to justify in any real sense on the planes (Vacuum, for instance - you'd have to be an absolute sadist of a DM to seriously send PCs there). So when 3E rolled around, the Inner Planes got 'simplified' by a wholesale ditching of the lot.
Which is a pity. Yes, you can have independent 'pocket' planes hanging around out there, with ice elementals and dust mephits and tsnng, but these don't have any established link to the existing Elemental planes and you can't have the 'border wars' that were such an established part of the 2E planes. OTOH, going back to the 2E system doesn't work well for me either - aside from the uneven flavor of the 2E planes, you have to explain why the paraplanes and quasiplanes don't spawn their own child parasemiquasiquaverelemental planes of Melange, or what have you.
Thus, a third option. Which is, IMHO, both a good balance and uniquely Planescapian.
In this system, the six 'primal' Elemental planes are the building blocks of all matter. They can be thought of as arranged over the surface of a giant sphere or die, planes on opposite 'poles' opposed to each other. Where two planes meet, you have a border; things get uncomfortably exciting here. And in a few rare places, you have a point, where three planes happen to meet. You can draw a ring around each of these points - how big is up to you, we're dealing with infinite planes after all. In these places, you get more than mere excitement. You get something... new.
Under this cosmology, you'd have eight elemental Demiplanes - each one combining two elements and either the Positive or Negative plane. (For a physical model, take a six-sided die and snip off the corners; the 'triangles' that result are elemental demiplanes, the main faces are primal planes.) Eight demiplanes is a manageable number; we can keep some of the more interesting paraplanes and quasiplanes, combine a few of the quasiplanes that were very similar to one another, and possibly add a new plane or two in the process. Here's some basics on how things might turn out:
- Fire, Earth, and Positive - Demiplane of Magma. This is the stronghold of the Azer; some of their kind have even been known to claim the demiplane as a Genesis creation of some wizard-hero of their people. Regardless of whether the plane is a natural or created place, the dwarf kindred are the dominant civilization here, using molten and near-molten rock as a tool for building and forging weapons and armor. It's still a frontier, however, home to thoqqua and many other beasts.
- Fire, Air, and Positive - Demiplane of Brilliance. This demiplane is a mysterious wilderness (combining all the dangerous exuberance of the old Radiance and Lightning quasiplanes) that few have explored. Creatures here include lightning elementals, scile, shockers, darklights, and the strange, giant ribbonlike creatures known as flickers.
- Water, Earth and Positive - Demiplane of Wood. This would be a new demiplane, filled with plant life of all kinds from great to small. Some cultures venerate it as a 'true' elemental plane, but sages in the Rational tradition call it a demiplane, drawing on water, soil and lifeforce to survive even as Material plane plants do.
- Water, Air and Positive - Demiplane of Tempests This is a plane whose exact identity and 'hook' I'm unsure about - my current placeholder is for a place of wild typhoons, waterspouts and tidal bores. Creatures and material would likely be new work. Anyone who has an interesting idea that would fit into this 'niche' of the cosmology, feel free to suggest it.
- Earth, Fire and Negative - Demiplane of Dust. This combines the old Dust and Ash quasiplanes, in equal measure, and has elements of both in place. Dust mephits are the main 'civilizing' force (though that's not saying much) and other creatures of dust and ash serve as predator species, including the rast.
- Earth, Water and Negative - Demiplane of Crystal. One might be surprised to see such a beautiful and brilliant concept as crystal linked to the negative plane - until one realizes that crystals trap light, breaking it apart, and in other forms trap and retard earth and water as well. This is the old quasiplane of Mineral (though with a darker and mre ambiguous slant) as well as the quasiplane of Salt (which is just as dangerous as it's always been). The would-be archomental Crystalle and his servants work to make the demiplane more benign, while the tsnng and facets seek to broaden its power at the expense of Earth and Water.
- Air, Water and Negative - Demiplane of Ice. Very similar to the 2e paraplane, with Cryonax, frost salamanders and all. More details might also be put into place for this location, as possible.
- Air, Fire and Negative - Demiplane of Smoke. Again, the 2e paraplane is the major source of information here; ideas and places from the old quasiplane of Steam might also find their way here. Aside from the Demiplane of Tempests, though, this is probably the one place with the greatest need for new work.
That's about the whole of things, then. Any questions, comments on how it might work, or ideas on how to change things or add detail?
The Elemental Plane of Wood is one of the stupidest things I read in the new Manual of the Planes. I despise it so much. And in your set up, it doesn't even really make a whole lot of sense. The combination of water and earth doesn't make plants (plants in earth use water), even with the combination of the life energy of the positive plane I really dont' think it works very well. I'd rather see it just as a random demi-plane floating in the ethereal if you woudl have it at all.
I really was a fan of the Planescape method myself. It seems to make alot of sense in most of the cases (there are a few weird ones that could be changed, either ash or dust needs to be dropped and replaced since they both have almost the same flavor). You can see how alot of these would be logical transitions from one of the main four elemental planes or the two energy planes to another.
I happen to really like the plane of Vacuum myself. There's something just so cool about it. It's like the ultimate example of the Doomguard philosophy, their goal. It's like exploring outer space (without the cold). It's a nice plane to oppose Radiance.
On to your Idea though . . .
Why would there only be new planes where two planes meet an energy plane? The thing I liked about the Paraelemental planes is that they were like natural transitions from one Big Elemental plane to another. In your set up, how is the transition from fire to earth different from the plane of magma?
I do like the possibilities for the Crystal Plane idea.