A "default" setting needs to be a fantasy world. Not Forgotten Realms (but I have little to no hope they wouldn't just keep up the branding and marketing for FR that they've steeped so many years into).
Exactly, the D&D movie, Honor among Thieves, guarantees Forgotten Realms is the default setting for 5e 2024.
There is no chance of any other setting.
But the core rules need to be useful for diverse official settings − and especially useful while actively encouraging DMs to worldbuild their own setting for the players at a particular table.
Exandria is a distinct possibility. And would appeal to a modern player base sensibility, I would expect.
Probably, the 2024 version of Forgotten Realms will incorporate inspiration from Exandria.
I, personally, would rather something more "generic fantasy."
This where the distinction between "core rules" and "default setting" comes in. The rules themselves need to be generic, setting-neutral.
A setting guide as a separate book, such as for Exandria, can go into more exacting detail to finetune mechanical rules for specific setting concepts. Classes in a specific setting can refer to specific in-world institutions.
More "Good vs. Evil" based. A forces of Good and forces of Evil being tangible, known. Have their godly entities, their in world champions, their kingdoms/despots/would-be world conquerors, and their enslaved/ensorcelled/or just plain made BAD adherrants/soldiers/minions.
At least for me, the 2014 alignment system, where it is a biographical narrative and never a mechanic, is working well enough.
I am uncomfortable with "gods" but comfortable with "celestials", that are angellike immortals that personify various flavors. These celestials have nothing to do with animism. But the celestials and fiends are interesting to explore the "Jungian archetypes" of the Astral Plane.
As someone said, "known places where evil comes from."
I avidly disagree with the "modern game culture" sentiment of "no inherently evil humanoid races." That's just ridiculous. The game requires cannon fodder. You're never going to make Level 2 worrying about if every kobold/bugbear/troll you encounter is another one of those "special individual" type monsters, with a soul and a conscience and a free will to choose goodness. Another chance to rehabilitate some creature looking to flay your flesh from your bones, burn your farms and homes, and bathe in their blood... if you could just give them a kind word and maybe a little food. Just show them how there's "a better way"....and you'll have a friend forever.
Every group of adversaries or plot a party encounters doesn't need to incorporate a half dozen different species just to make everyone "feel good" and pretend "not all goblins (orcs/drow/what have you) are evil." Every species has its enlightened singularities who are balking their evil society/system (or just saving their own skins) and gonna show you how everyone (ergo, noone) is special.
I have no interest in a game where the only things to worry about are beasts, goo's and blobs, or full out "aberrations." So we/players don't need to feel "bad" about slaying "humanoid" make believe monsters.
Because every humanoid "race" relates to reallife ethnicities, directly and indirectly, to demonize a Humanoid species inevitably runs afoul.
I am comfortable with certain "factions" supplying the canon fodder. Not every Orc, but the members of a certain Cult of Gruumsh can be "typically" Evil. Same goes for certain Human factions, like a fantasy version of a drug cartel or other tyranny can be "typically" Evil.
Magic needs to be well thought out, with areas of more or less readily available magicality. What might be a "common household item" in one region seems a remarkable magical treasure in another. The differences between studied sorcery, god-granted divine magic, natural primordial/elemental energies, demon-/fae-/dragon-pacted occult powers, and innate psychic abilities all need be represented and differentiated within the world to supply more flavor (and hopefully mechanical support) to make the various casters all "feel" (and play) differently within the setting.
Good point about different cultures having different access to technology, as well as developing alternate kinds of technology.
ummmm...what else....Whatever the default setting, keep your technology OUT of my fantasy. Firearms, clockwork, steampunk, whatever... If there are "machines," or "factories" or "air ships," "mass transit," etc... it should be based on magic or the use of fantastical creatures. Make "tech" a supplement/add on for the game type of thing. Not a default setting thing. ...OR as in my homebrew, things that are from lost/forgotten/layered upon civilizations that no longer exist, "indistinguishable from magic."
For core rules, there is less difference between a
Fireball and a grenade.
Each setting flavor determines the feel.
The default setting − Forgotten Realms − is medievalesque. It tends to be historical medieval, or comparable to it. Its advanced technology is magitech.
However, its social structures tend to be modern, such as gender egalitarianism. Cities that include diverse ethnicities from around the planet tend to be the typical gaming experience, even tho there are more isolated regions that feature a specific ethnicity.