I'm amused by how little of what you fell in love with is part of the original Great Wheel. The Feywild and the Shadowfell are pure 4e/World Axis, as for that matter is the Elemental Chaos as opposed to much purer elemental planes. Mechanus is a substitute that doesn't really fit - but is so much stronger than the original Nirvana that no one cares. And Ysgard is just off-brand Asgard, not that there's anything wrong with that.Exactly this. When I first looked at the Great Wheel Cosmology in 5e, I thought "Oh, cool! Look at all of these planes of existence! I'm sure that in the near-50 years of D&D's history that these have all been well-thought-out and detailed to have them included in campaigns, especially as places for the PCs to journey to!"
Then I spent about an hour or two reading about the planes in the DMG. I quickly fell in love with a few of the planes (Mechanus, Limbo, the Feywild and Shadowfell, the Astral Plane, the Elemental Planes, the eternal war aspects of Ysgard and Acheron, and a few others), but also began to realize that a lot of the planes were lacking in description and/or largely redundant with other planes. I really liked the idea of Carceri as "the Escape Room to End All Escape Rooms", but it barely had any description in the DMG and other official sources, so I never really ended up using it (I used it one time, and it was because of an extremely unlucky roll from a PC against a casting of Prismatic Spray).
7 Heavens is so someone can say they are in 7th Heaven. The Nine Hells (not seven) started as the Nine Circles of Hell in Dante's Inferno.also got a few questions, like "Why the heck are there 7 different flavors of Heaven and 7 others for Hell,
Half of that's in the Feywild or Shadowfell. Why there wasn't before 4e? Pass. There's a reason fluff wise I consider it goes 4e> 5e > everything before.but no Plane of Mirrors, Dream/Nightmare Realm, Death World, Twilight Forest, or . . . [etc]?",
The difference between a lightly satirical version of Narnia and Faerie."What's the difference between the Beastlands and the Feywild?",
Slaad made it to D&D first."Why are the embodiments of pure chaos [Slaad] evil, multicolored frog-men and not Fey?!?!",
Different approaches. In Valhalla war is almost fun. Acheron is much grimmer. This is one I can actually see. Of course I once again prefer the 4e World Axis domains of war because one invaded the other."Why do we have two different planes of War? [Ysgard and Acheron]",
Pass"Why isn't there a plane of Dragons and another of Giants?",
Because Olympus isn't supposed to be infinitely big. Mount Olympus is in Arvendor (CG)"Why isn't there a Mount Olympus Plane if there's a Hades,
No idea at all here.and why isn't Hades the Underworld!?!?",
If we have Neutral Evil Fiends (itself a little box-ticky) they gotta have a domain. But no idea other than they wanted to put the anchors for Mount Olympus and Yggdrasil right at the bottom of the wheel."Why do we need a plane for Neutral Evil fiends [the extremely boring and uninspiring Yugoloths], and why is the plane for Neutral Evil fiends not the Neutral Evil Plane of Existence [Hades]!?!?"
There is. In 4e it's called the Astral Sea and in 5e it's called the Great Wheel and goes on the outside of the axis.and "Why the heck are all of these redundant planes of existence different from each other, and not just separate parts of different, bigger, and more interesting plane of existence?"