and now you are sounding just like me
How do you try to capture the diversity of all the different worlds in a single example. The answer is, you can't. But you can work to keep some consistency within the expressions. If you've got a video game, a comic book and a TRPG product all set in FR, then keep the vision consistent - so those folks that love it, get the same experience no matter which expression they interact with. If you've got a video game, and novel and an animated short set in DL, then do the same thing. A product that goes across all the worlds, like the core rule books??? That's a good question. I had actually proposed to R&D that we do the core rule books without art at one point. So that they could be setting and world agnostic. That didn't set well with most folks. So now, they have to decide which setting will be used as the point-of-view, so that I can choose how to depict the monsters in the MM.
There are worse people to sound like.
You thought about being "setting and world agnostic." But did you think about going "setting and world polytheistic?" Rather than the message being: "We won't tell you what it's like!", the message can be "We will let you pick from LOTS of different ways it can be!"
So to represent that visually in the books, rather than no art and no context, have incompatible art and contradictory context. Orcs from FR meet goblins from Dragonlance meet psionics from Dark Sun. There's 4e comic-book-y art and two pages later is 2e fabulous-80's-hair art and the next chapter contains something like 3e WAR-y art and a 1e-esque black-and-white tableau. On one page there's a vision of a desert world with a thick, dark sun, on another page there's a scene in a verdant pulpy jungle temple, and on a third there's a dark, Gothic castle shrouded in mist, and then there's some sort of pseudo-Asian pagoda with modrons marching out of it done in the style of an old Japanese watercolor or something. You can play all these worlds in the same game, but none of these worlds defines the "default" game, because the default game is what you make with your table.
You don't have to provide stats for every type of goblin up front, but you can pick from the entire array of potential D&D goblins, and present one (while perhaps referencing alternative possibilities), and know that this kind of goblin isn't "the goblin," but just one amongst a menagerie of goblins. It's not the goblin all media is to use, it's one goblin that media can use.
Maybe think of each book as something like the 2e MM: the book starts with space giants from the '70's and semi-sci-fi oozes and ended with psuedo-arabian turtle-islands and pulpy psychic jungle snake-men and...frickin' zombies.... That was an accident of history, jamming all those disparate critters into the same massive tome. But it was
insanely effective at communicating the idea that D&D was a home for any kind of fantasy you can imagine. It's something I think NEXT, with its message of modularity and flexibility, is well-suited to embracing again. There should be pictures of elves in spelljammers and halflings in jungle gear and lightning rails: D&D is all of that, too.
The message of diversity, inclusiveness, and infinite imaginative possibility, isn't as effectively delivered when there's a consistent look for goblins (or whatever) across the whole of the game, because that's saying "there's only one kind of goblin in D&D." There should totally be one kind of goblin in FR, one and only one Drizzt (with his cape and his scimitars and his flowing white hair) but that doesn't need to apply to the whole game!