Micro-elementals

If coterminous or external elemental planes don't appeal to you, here's another idea that takes them in a different direction than the standard view or my previous posts. If we take the classical view that the four elements are the fundamental consituents of all matter, then (as science fantasy has told us at least since Ray Cummings' The Girl in the Golden Atom in 1919) there may very well be worlds inside those tiny particles. Elemental worlds.

Like Microworld, yeah, except that unlike its organic chemistry model look, elemental microworlds look like the platonic solids just like Plato told us they would. 

Maybe the world is on the interior of these shapes or maybe on their strangely-angled surfaces. Either way, they would be pretty weird places. Of course, this also may mean that elementals are microscope--even atomic level things. An elemental summoning would actually be growing a fractional bit of element to macroscopic size. The fact that size creatures have (rudimentary) intelligence might suggest that these microplanes themselves are intelligent. The implications of that, I'll leave you to contemplate.