Don't get me wrong, I find a lot of the in-between planes interesting (I like Acheron and Carceri), but Pandemonium could easily just be a few layers of the Abyss, and if Gehenna is the home of the Neutral Evil Fiends (Yugoloths), it should be the Neutral Evil Plane, instead of Hades (which isn't even the Underworld/Afterlife of the Great Wheel, it's just a place that Larva form and Night Hags have soul-trades). (And, like I said before, there's not really a reason to have two separate Planes of War, so merging Acheron and Ysgard and having the specific conflicts on them be separated by layers on the same plane, like how the Nine Hells has 9 Layers, etc.)
And don't get me started on the Upper Planes or Limbo.
(Sorry for the rant.)
Personally, I have no problem just ignoring the Great Wheel and stealing the bits I like. I had created a "minimal cosmos" (which, it turns out, had some similarities to 4e's World Axis, although when I made my cosmos I hadn't actually
known anything about the World Axis). I wouldn't mind if there was just Hell (or the Abyss, or whatever name you wanted to give it) and the only thing that each layer had in common was that it was Evil. It wouldn't matter if a layer was CE, LE, NE, C(N)E, L(N)E, N(L)E, or N(C)E. And ditto for Heaven (or Celestia, or whatever) and for whatever took the place--if anything--of the True Neutral planes.
Or heck, go back to the beginning and have planes of Chaos, Law, and Neutrality as the basis, with individual areas or layers within each plane that tend towards Evil and Good.
There
is a decent difference between Ysgard and Acheron, though: Ysgard is (or at least can be seen as) the plane of noble war, or at least of glorious battle--sort of a fantastic ideal of what war is--and Acheron is about the horrors of pointless war, with no true triumphs or actual heroes (and probably closer to the reality, only a lot worse).
Maybe this could be a single, two-layered plane--kind of the inverse of Bytopia, where the two layers are on either side of a planar coin instead of being the filling of a good-aligned sandwich cookie--but they have enough of a different feel to me that I don't mind them being separate places.