The name Great Wheel itself comes from PS if I'm not mistaken. IIRC, the 1e PHB had the planes in a box, not a circle.
A square in the PHB; a circle in DDG. And as far as I know you are right that "the Great Wheel" is a Planescape term.
I sooo like the name Outlands better than Concordant Opposition. Sounds like something a sage would call it instead of someone whose actually been there.
Concordant Opposition is a really clunky name. Wasn't it Jeff Grubb that came up with it in MotP rather than Gary? Not sure Outlands was great choice for a plane that was put in the middle of the other outer planes though.
To the best of my knowledge, "Concordant Opposition" first appeared in DDG - author credits are James Ward and Rob Kuntz. In Gygax's PHB there was no "true neutral" plane, which I think fitted with the idea that clerics couldn't be neutral and only druids could. But all the neutral gods in DDG obviously needed a home, as did Boccob and the other Greyhawk gods with true neutral clerics!
I prefer Concordant Opposition to the Outlands as a name for a plane of true neutrality - the former, but not the latter, actually conveys something of the thematic idea of the plane. The fact that is sounds slightly sage-y or
outlandish (!) is a virtue, in my view, for the name of an outer plane.
On the other hand, especially for monster descriptions, I want it to be as detailed as possible. If you don't use Planescape lore, you're senselessly ignoring tons of detail that has been added over the years.
If there's only a little detail, then every DM has to come up with details for every monster they want to use. See the problem there?
Personally I don't see the problem. And I don't want as much detail as possible. For me, a big part of RPGing is (together with my players) creating my own fiction, rather than immersing myself in fiction that someone else already created. Monster details that are suggestive of trope and theme are good; monster details that answer all the questions before play even starts, less so.
If they commit to carrying forward established lore, they shouldn't contradict established lore.
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the Planescape Slaad is the "core" D&D Slaad (actually, the only D&D slaad). OD&D and 1e (the Greyhawk default setting) said "these monsters are from the Outer Planes." Forgotten Realms had those creatures, and said they were from those same Outer Planes. MotP and Planescape came along and said "You know those Outer Planes we're always talking about? Here's more details about them and the creatures that come from there." The Planes were never a separate setting.
Planescape didn't create its own new versions of these creatures; it built on the lore that was already there.
I personally don't care about the semantics of "new" vs "building". The point for me is that, having mostly skipped Planescape, when I picked up 3E planar stuff (eg the MotP) it was full of this lore that I didn't recognise from my old MMs and my old (Jeff Grubb) MotP.
For me, that lore is new, it's at odds with my game, and it's not really something that I care for. The fact that it doesn't
contradict anything in my old books is neither here nor there for me; it contradicts what
I was doing with those books.
Don't "repurpose" monsters, because then you are taking something away from the people who liked the previous version.
OK, but this applies equally to Planescape, which "repurposed" the daemons from D3, the DMG, Fiend Folio and MM2 into these "yugoloth" things that I don't really recognise. Why is that repurposing permitted, but other repurposings out of bounds?
From a Planescape perspective, the core game's vague descriptions represent what "everyone knows" about the Planes, and then the Planescape product goes on to show that it's not like that at all.
This is a big part of why I don't like Planescape. I don't mind backstory, but I dislike backstory that invalidates the known lore and tends to make PCs (and their players) look like fools. In Planescape, I find that this tends to be further combined with a slightly annoying form of cynical relativism.