I love Planescape as a D&D setting.
I come to fantasy for the weirdness, the otherworldliness, and Planescape invites you to play D&D in settings that take you far, far away from familiar, mundane reality, even more than the usual lands of wizards and dragons.
But it does create this odd issue:
As I've written about before, the influence of some outer plane in a campaign set within the Prime Material Plane (as most are) is one thing: a villain's entire plan might be to allow the searing fires of hell to be unleashed upon the world, and in such a campaign, the way that the Nine Hells would likely be portrayed is as some unimaginable nightmare of fire, spiky metal, and tormented souls.
In a Planescape campaign, the Nine Hells is likely to be... well, yes, filled with fire, spiky metal, and tormented souls, but also shops and inns and NPCs you might do quests for.
Planescape allows us into these outer planes, these truly separate planes of existence, but in doing so, forces us to portray them as at least slightly familiar to our regular reality.
I wrote recently about my desire to start a Planescape campaign on the Plain of Infinite Portals, the first layer of the Abyss, and specifically have it start off in the "Worst Bar in the Multiverse." The prompt for each player is to ask what mistakes they made in their past to wind up in such a place, and then have them adventure across the wasteland, in a kind of Weird West/Mad Max-like environment filled with demons.
But again, there's a version of the Abyss that ought to just be endless incomprehensible horrors, like a constant nightmare, where even glimpsing it might drive one insane.
And this got me thinking:
Planes have depth.
There's an idea in those 2nd Edition Planescape books (I think? Come to think of it, I don't have the corresponding PDFs) where the Elemental Planes can be visited by mortals, but only in the shallow depths of them - the elements mix enough in the shallows that, for example, there's air to breathe, or earth to stand on, even if you're in the Plane of Fire. But that the deeper you go into it, the more pure it becomes, and basically if you're not an actual elemental of the corresponding element, there's no real way to exist there in the deepest parts.
I think this can apply to every plane. Except, maybe, the Prime Material Plane.
Funnily enough, in my homebrew setting, the denizens of the connected part of the Shadowfell and Feywild (those parts that overlap my world) refer to the Prime Material Plane as the Flatlands. And while that was just a way of saying "it's kind of nondescript or boring," I actually think it makes a lot of sense if we think of the Prime Material Plane as not having any layers or depth - once you're in the plane, it just works according to some fantasy approximation of real-world physics (I tend to say it's real-world physics unless I call out a specific exception).
In Baldur's Gate 3, the second act takes place largely in the Shadow-Cursed Lands, which have been touched by the Shadowfell. But we do, on a major quest line, go to the actual Shadowfell, and it's portrayed as a swirling vortex of shadow, with no realistic way it could be inhabitable.
The funny thing is that the Shadow-Cursed Lands look more like the way that I'd portray the Shadowfell itself in a game I ran. But what might BG3's Shadowfell be equivalent to (not counting Hades, which does, to be fair, share a lot of vibes with the Shadowfell).
The answer, I think, is that that's "deep" in the Shadowfell, whereas the just dark, spooky version of the Flatlands (it's so much quicker to type than Prime Material Plane) would be the shallow part of the Shadowfell.
And you know what's beautiful about this? It accounts for the Domains of Dread.
While the shallow Shadowfell is the weird mirror world to the Flatlands, the Domains of Dread are deeper in the plane, perhaps not strictly corresponding to any real location in the Flatlands, but resembling them until you notice the foggy border beyond which there doesn't seem to be anything.
Unlike the "Border Shadowfell" (much like the Border Ethereal in contrast with the Deep Ethereal - see, I'm not making this up out of nowhere), the Domains of Dread are a little farther in, a little farther from familiar reality as we know it, and a little deeper into the swirling darkness and mists of nightmare logic. Perhaps deeper still than the Domains of Dread is the kind of swirling endless darkness in which gods like Shar or entities like the Dark Powers reside.
Now, the Outer Planes, at least, have what's called Layers. I think the only outer planes without them are The Outlands (the true neutral plane, which most resembles the Flatlands until you notice that each element of the landscape is perfectly balanced - a deep ocean for every mountain, a frigid tundra for each burning desert) and Limbo, the Chaotic Neutral plane, where any such structure would be anathema to what the plane stands for (arguably it either has just one layer or a constantly shifting number of them).
Planar layers are a little weird - on a certain level, it allows DMs to cordon off certain parts of the plane or focus on particular regions and vibes. Often, the deeper one goes into a plane (though in the case of Mount Celestia, unsurprisingly, the "deeper" layers are farther up the mountain) the more extreme the plane's whole vibe gets. For example, in the six layers of Carceri, the first layer, Orthrys (named for the mountain upon which the Titans of Greek myth lived, in contrast with Mount Olympus) the string of planetoids are close enough that there are structures built between them (including the palace of the aforementioned Titans - another name for Carceri is Tartarus, also from Greek myth). However, on the deepest layer, the planetoids of the plane are so far from one another that you can't easily see the next from the surface, the sense of profound isolation taken to an extreme.
But in a certain way, I think that layers and depth might not be quite the same idea: for instance, the Abyss is reputed to have infinite layers (far more than any other plane) though some argue this is just that no one has been able to count them all. Various demon lords have domains that take up one of these layers (though some, like Juiblex and Zuggtmoy share a layer, while Grazz't has three layers all to himself and his minions).
These layers are given numbers, indicating their relative order, but this order is somewhat arbitrary. While the Nine Hells of Baator and the Seven Heavens of Mount Celestia both prevent planar travel to anywhere except their first layer, forcing you to traverse all the layers between you and your destination on the plane, the Abyss requires no such travel - the Plain of Infinite Portals, the first layer of the Abyss, is filled with sink-holes that let you fall down into other layers directly (I really like making anything Abyss-related, like demonic temples, involve a deep, vertical shaft that one must descend).
Demogorgon's layer, the Gaping Maw, is not the lowest layer of the Abyss, even if he's (they're?) generally considered the mightiest of the demon lords.
So it's not a perfect, direct correspondence between the two concepts.
Now, there's an alternate way of looking at this:
Player characters are, generally, mortal beings. While the creature types that you can play as have expanded considerably over time (Forge of the Artificer gives us our first official playable Aberration by redesignating the Kalashtar, and we also get Constructs and Fey in the Warforged and Changeling, respectively) I think we're generally meant to play these characters as being people first and weird monster second.
Thus, a way you could play the Outer Planes, or even just other planes in general, is that our perception of them is not necessarily what they are. We might see Avernus as a giant, blasted wasteland with an endless war raging across it because that's the closest comprehensible equivalent of what's going on that we can imagine. What does it mean for Lawful Evil and Chaotic Evil to clash (in a realm that is the home territory of the former) in a philosophical sense? Well, that's hard to visualize, so instead we see it as clashing armies of regimented tyrannical brutes fighting slobbering hordes of pscyho-killers.
Even fiends themselves might not truly look the way we perceive them, because it turns out that the impulse to pursue one's reckless ambition at all costs doesn't really look like anything - except in this fantasy world, where we see it as a hulking demon with giant pincer arms.
Still, I think you can get a lot of mileage about thinking of these planes as appearing more like reality on the nearby shores, but as one delves deeper into them, the experience becomes more impressionistic, more oneiric, more abstract.