The core can easily be setting neutral enough.
It already is.
Using just the PHB, I can play Forgotten Realms, Greyhawk, Mystara*
, Ravenloft, and Exandria. With a handful of additional options, I can play Eberron, Ravnica, Theros, Dragonlance*, and Birthright**. The elf fighter I roll up in the PHB currently is 90% flavor compatible and 100% rule compatible with all of these setting. There is only one official setting which bucks that rule: guess which one it is.
* Going by the 3e version of the setting, with a few exclusions on race.
** Since these settings haven't seen a single item since 2e, there is more guesswork here, but its safe to assume there is no conflict with the post-2000 races and classes.
Core rules do well to avoid any cosmological assumptions.
Especially, religions need to be more diverse and customizable in D&D, more like Eberron - so reallife religious minorities can feel more welcome.
And religious diversity makes it more convenient to world-build homebrew settings that have special assumptions.
The DMG has a whole chapter on different belief systems. The gist is spelled out again in Xanathar in a sidebar, as well as a the druid sidebar in the PHB.
D&D is built around fantasy polytheism, with some allowances for other forces, philosophies, or beliefs like animism or ancestor/spirit worship. Several different pantheons are presented in the back of the PHB (and another in the DMG). There is no single religious belief, no universal source of power, no All-knowing/All-Powerful Monothestic God unless you want there to be. This is a literally what you want: a set of options that the setting determines use on, with multiple examples for the DM to choose which fits. You want something where Gods are mysterious and unknowable? Go Eberron. Want something where Gods come to your house for tea? Go Theros. Want the Gods to be dead? Welcome to Athas.
I can't see how this can be any more inclusive.
With regard to classes, setting-neutral classes and archetypes make it easy to cut-and-paste the concepts that are relevant and to leave out the ones that are less relevant.
Especially the basic classes - Cleric, Fighter, Rogue, and Wizard - need setting-neutral mechanics, to plug-and-play into a diversity of homebrew settings.
First off, I fail to see what is not setting neutral about the Iconic Four: they are as dull as dishwater when it comes to flavor. Second, where is the bard, druid, ranger, paladin, sorcerer, warlock, barbarian, and monk all going? As I stated early, they are known and playable in ALL official D&D settings save one. Are you going to reprint them in every campaign guide? Make people shell out for a 4th (5th?) book of options? (which puts us back in the same boat as them all in the PHB, except now it costs $100). Maybe put one or two per campaign guide and make people buy the Greyhawk book for Druid and Paladin, the Forgotten Realm book for Sorcerer and Barbarian, the Ravenloft book for Bard and... you get the point (cue the riots at buying dozens of sourcebooks to get the full set of classes, 4e was bad enough with 3 PHBs to get all 3.5 classes).
In sum, setting-neutral class descriptions for the core rules makes it easier to plug and play into Dark Sun flavor, ... and also into any homebrew setting flavors.
No it doesn't, it just disperses the work to other books and raises the buy-in cost buy adding more books to get the complete game. Further, it does nothing to help homebrewing because by definition homebrewing is an infinite variety of changes. A setting-neutral PHB isn't setting neutral if it still assumes...
- There are other races beyond human
- Technology resembles late-medieval Europe (Rapiers, full plate, crossbows, spyglasses, galleons)
- Magic is something that can be wielded by mortals safely
- Magical effects like raising the dead, scrying, teleporting, mind-reading, summoning, necromancy, and healing are even possible
just to name a few core assumptions that are frequently changed to fit the settings of homebrewers. There is no PHB that can be reductive enough to support Lord of the Rings, Harry Potter, and Game of Thrones.
Setting neutral core rules helps ME! world build.
Good for you. A lightly flavored core (such as that which existed in 2e and 3e) helped me. I didn't need the core rules to tell me it was ok to make my elves different or remove orcs or add ninjas, I just did it and told my players "the stuff in the PHB is wrong, read this".
Not sure how I did it all those years ago. Heck, not sure how I do it now when a player asked to be a drow in my Eberron game and had to go "actually..."
Races/species need to be far more customizable anyway. The customizability helps a player feel more comfortable about reallife concerns about ethnicity. The same customizability makes it easy for the DM to tweak it to make it more resonant for a different setting.
Rule 0. If you don't like it, change it.
But don't couch your desire for bland, flavorless SRD-like core rules to help say no to your players while creating your homebrew with any sort of concern for inclusiveness. People need better representations, not none-at-all. This isn't Generic Fantasy Simulator d20. Its D&D. And most of D&D works fine with lore attached to it. Some of the lore might need updating, but don't use that as an excuse to get rid of it all in the name of "inclusiveness". Nothing will drive people away faster than a game that says nothing and stands for nothing. Even Paizo wisely uses lore to define its game, and it is one of the most inclusive variants of D&D today. (Mostly; about those ability score perks/flaws).
The goal should be to make better elves, not strip elves of all meaning and reduce them to a visual skin a few mechanical perks.