Fey get their own plane in the Feywild and fiends get to choose one of 3-4 hells..
What would be the plane of D&D giants be? If hill, stone, frost, fire, cloud and storm giants were shunted off to their own plane, how would the D&D version of it look? Straight out of Norse mythology? Like the Material plane but everything bigger? Resource poorer to encourage extraplanar raiding? An outer plane, inner plane, or parallel plane?
Chthonic. Giants have their own rules, ancient and mysterious. Their home is steeped in deep, dark magic, and they, its rulers, are both powerful and a little bit alien--but not so alien as to be too far away. Think of it as...sort of "perpendicular" to the Feywild/Shadowfell divide, pushing along a different axis, one that draws more on vast, unknowable things: not fairy-rings, but the Forest Primeval; not graveyards, but the Standing Stones and the Treacherous Moorlands and the Mountains Eternal. Neither dark nor light, but rather the
bones of the world risen (or raised) up.
Draw in more Norse myth--things like their plane being the repurposed body of some ancient progenitor-giant, having siblings that are huge monsters, chthonic rituals and blood-sacrifice to keep the world alive. Perhaps draw in some bits of other mythology that courts this kind of thing too, like Maya or Aztec, where the world itself may be a Lovecraftian horror that must be fed or it will awaken.
(And, as an aside, the logical opposite of Giants also fit in: Dragons. Where Giants come from the chthonian darkness, Dragons come from Olympian heaven, having the power of
flight. Each side is elemental, but in radically different ways. Tiamat, in this conception, may have done her great evil not by fighting Bahamut, but by trying to take absolute power through an "unholy"
alliance with the Giants--or perhaps that was her Start of Darkness, and she consorted with steadily more wicked powers after her Giant allies failed her.)
What about a plane of beasts? A plane dominated by animal instincts and habitats. What magicall effects would be there? Which gods would move to the plane that has infinite puppies? Is that were the druids get all the summoned animals from?
I...have a much harder time with this prompt. "Beasts" is a bit too generic, particularly with the Feywild already in play, and the above covering most of the "megafauna" possibilities. I'd really need to sit down and think about what a "beast-plane" would center on.
Is Oozeland just a plane of jelly and jam with different levels of corrosiveness and sweetness?
I'd go with an idea I've seen elsewhere, and basically treat oozes as a manifestation of the Far Realm bubbling up through the Deep Underearth. They have no discernible biology because they're literally living by an actually alien physics to "our reality" (the mortal plane or planes)--which is also why they never get to the surface, because that's
too far from their "home," and their nature can't handle that much of "normality." Things that mimic true sunlight are thus, in a sense, forcibly making the Deep Underearth more like the "normal" world--it's not that UV light kills them, it's that forcing them to exist in the rules of "our world" means...they don't get to exist anymore.
Is Mechanus the plane of constructs?
Mechanus is the plane of Order In Motion. It is not static and unmoving, an ossified husk, like some would have you believe Total Order would be. To be static is to invite chaos, because you cannot restore things to their place if disturbed. So Mechanus is a place of gears and timing, of the beauty of perfect symmetry, of laws that admit no exceptions because no exceptions are
needed. Its denizens, then, are representatives of Order In Motion, beings that move with unnaturalness that transcends into something "more" natural (what the Japanese would call
yugen.) None of this weird clickety-clackety rickety silliness (except for things that have gotten too much Chaos in them); modrons and inevitables are well-known, but they should include geometers, astronomers, philologists, lawyers--a caste that is more priestly because it is more scientific and more scientific because it is priestly. And even non-sapients should be brought in. Social insects are one obvious option, but so are migratory creatures like birds and butterflies, trees that follow neat and clean seasonal cycles, things like shellfish that slowly and steadily grow greater shells.
Mechanus is the Druid's worst nightmare given life: a world where Order-In-Motion is totally, and obviously, unnatural....and yet woven through every leaf that falls and every eye that sees.
As noted above, I'd want a plane for Dragons. It would need to be
super gorram metal, but I haven't thought about it in enough detail to specify exactly how yet. It should be scary and dangerous but full of power--the kind of place that trees that grow for a thousand years and produce one magical fruit a century grow, where wuxia antics are
ordinary combat and the differences between "skill," "power," "magic," and "awe" are
exclusively pedantry.