megamania said:
I have always looked at the elemental planes as 85% of said elemental. The other 15% is a means to enhance the elemtals.
You can look at it this way, but you don't really have to. You can have 99.9% of the plane composed of one element and still have a very varied plane.
Since we are talking mostly about the plane of fire, consider the Fire elemental. We know that fire elementals don't have the incorporeal subtype. Furthermore, we know that they have a strength score and can directly lift and move solid objects. It's perfectly reasonable to suppose that fire elementals are in some sense 'solid'. This gives us canonical evidence that fire can be arranged in ways that give it different properties. In some the fire 'atoms' join up to make 'molecules of fire' that have gaseous forms, and in others they apparantly make solid forms. Perhaps on the plane of fire, fire atoms can achieve a density that effectively make them a solid substance? Perhaps it's a difference in structure which causes the change in property much as in the real world different structures of carbon give us things as different as graphite, benzene, and diamond. But whatever you decide, there is no canonical reason to assume that the plane of fire is empty, particularly considering how densely inhabited the plane is (Azer, Efreeti, Salamanders, Elementals, etc.). Fire can be assumed to be hot and destructive, and that its never wet, but beyond that it probably varies alot. Additionally, we know that the plane of fire contains relatively large amounts of smoke, ash, light, and lava (even if its just 1 cubic foot of every 1000), and smaller amounts of stone, certain metals (notably brass and its constitutients) and hot air. From this we can conclude that there are all sorts of terrain on the plane of fire. We can also assume that elementals can occur in nearly as widely different forms as living organism occur in the real world. So, there could be living 'fire plants' - trees, flowers, etc. - especially firey variations on those plants on the prime which require fire to distribute thier seeds and other plants with an tangible or mythic affinity for fire (holly, cherries, for example). There would be living fire animals: fire lions, flame lizards, flame snakes, salamanders, etc., as well as the more usual humanoid fire elementals that you typically summon. All of these things are available through application of templates.
So what do we have? A highly 3D world of varying moving dangerous terrain filled with exotic creatures and cultures. If you can't adventure in that its because it doesn't appeal to you, you can't get your head around it, or you think that it ought to be reserved for higher level challenges rather than just gifting the players with planar adaptation items and guides so that they could survive there.
None of this requires changing anything about the existing cosmology. It's just a matter of organizing what's already been said and laying it out.