I think the multitude of outer planes, along with alignment in general, are pretty silly. It's not not enough to have one heaven and one hell, D&D has to have at least three versions of each and the distinctions between them are arbitrary, random, and/or anal-retentive.
I think it would make more sense to use a more Moorcockian-inspired cosmology. This would exist on a continuum between law and chaos, without good or evil being distinct cosmic forces. Law (in combination with neutrality/cosmic balance) is represented by archons/angels, chaos is represented by demons. Devils are creations of law intended to combat the demons in their own realms and represent an extremely hostile form of order, with their inspiration being stuff like the Hellraiser comics. You don't need anything more than that, and prior concepts like modrons/psychopomps and slaad/proteans can be folded into angels and demons, respectively. (I know 4e did something similar and got blasted for it, but I think in this case they actually had the right idea.) Good and evil are no longer distinct cosmic forces but effects: good is the result of harmonious balance, whereas evil is the result of imbalance in either direction.
As for the inner planes... fey and elementals shouldn't be distinct creature types imo, especially since the idea of elementals was originally codified by Paracelsus and meant to classify fairies (and quite frankly you could apply an elemental association to any magical monster). I think fairies can exist as their own thing distinct from angels and demons, personifying the natural worlds that arose as the result of the conflict between order and chaos, altho obviously the specifics can vary by setting. The inner planes I'd reimagine as Fairyland, roughly divided into the Summerlands for the Seelie Court and Winterlands for the Unseelie Court. The actual environments could replicate any of the absurd locations from the original elemental planes, such as volcanic wastelands inhabited by fire nymphs, undersea kingdoms peopled by merfolk, cloud cities full of winged folk, underland peopled by anthropomorphic playing cards, primeval forests ruled by beast lords and plant lords, dark shadowlands haunted by shadow fey and specters, etc.