I also like this idea. One way to do it would be to get rid of the idea of distinct planes. Instead, the Inner Planes become a continuum, with six elemental "poles" of pure Earth, Fire, Air, Water, Positive Energy, and Negative Energy. As you move away from a pole, the pure element becomes increasingly mixed with others.So, the traditional elemental planes would be the regions close to the poles, where one element vastly predominates and there are only scattered intrusions from the others. The "border planes" Mike is talking about would be farther from the poles, where it's a pretty even mix but one element is noticeably stronger than the rest. The Prime lies at the mid-point, equidistant from all six poles.
This offers a handy way to incorporate the Shadowfell and the Feywild; the Shadowfell is the "border plane" of the Negative Energy pole, and the Feywild is the "border plane" of the Positive Energy pole. Likewise, the para-elemental and quasi-elemental planes would be a natural consequence of this arrangement. And you could have quests built around stuff like a journey to the uttermost heart of Elemental Fire.
Another benefit of doing things this way would be to make planar travel more interesting. Instead of just casting plane shift and pop! you're floating in the Elemental Plane of Air, you have to actually, you know, travel. Plane shift gives you the means to move along the planar continuum, but you have to move through space at the same time, much like shadow travel in "The Chronicles of Amber." (The traditional leap across planes would be reserved for those who can cast gate.)
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