Like you said, that'd be Nentir Vale 2.0. That world was a flop.
The Multiverse is inherent to D&D's richness. It'd be like smooshing all the worlds of M:tG into one planet. The "Multiverse" concept just made a huge leap into the mainstream with the recent Spider-Man: No Way Home film. People can handle diverse timelines.
If I were head of the D&D team, for 5.5E or 6E, I would go for a very specific two-prong articulation which was both more generic and more specific:
1) I would make the creation of a homebrew world simply part of the game. I would "gamify" that process by making a World Builder's Guidebook (WBG) a core rulebook (equal to the PHB, DMG, and MM); either that, or fill most of the DMG with that. This WBG would have randomized tables which would truly cover the entire array of world-features which have ever been seen in any D&D product, including Spelljammer. (Of course with the option to intentionally choose features instead of rolling.) So that "rolling up a world" is as much a part of the game as "rolling up a character." There'd even be a World Building Sheet, like a Character Sheet, but for worlds.
There'd even be a random "campaign setting name" table which includes (among many other results) all the name elements that make up the published worlds: e.g. "Grey", "Dragon", "Forgotten", "Dark", "Raven" + "Hawk", "Lance", "Realms", "Sun", "Loft", etc.
D&D Beyond would support World Building in the same way that it supports Character Creation.
AND, AT THE SAME TIME...
2) I would really coherentize the D&D Multiverse in all its diversity, as a single meta-setting.
Through introducing world-hopping as a common feature, even at low levels. Via alternate world gates and cross-world organizations, such as a Multiverse-spanning Adventurers League.
By producing official conversion notes which place every adventure in every published world.
By publishing an Atlas of the D&D Multiverse that provides the phlogistonic and planar equivalent of the Star Wars Galaxy map (which WotC designed), showing exactly where every D&D world and plane ever mentioned, in any edition, is located. And including official world maps for all the D&D worlds, from Toril to Krynn to Mystara to Oerth to Nerath.
By producing a Grand History of the D&D Multiverse, which finally synchronizes the timelines of all the published worlds.
By turning all continuity discrepancies into alternate timelines. And giving those parallel universes ("paraverses") official alphanumeric designations (like how Marvel and DC name their alternate earths).
This Multiverse would serve as a setting for cross-world novels and media events. People can handle it. The M:tG and MCU and Spider-Man films are all introducing the general public to the complexities of alternate worlds and timelines. D&D RPG folk ought to be able to handle it.
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A two-pronged approach. Both more generic and more specific.
My fear though is that this approach doesn't create a "first door" for people new to the game to walk through.
My first D&D was Mystara, though I didn't know it at the time. My friend ran material right out the classic BD&D modules, the Gazetteers, and the Rules Cyclopedia. When it became my turn to DM, I followed his lead. As I grew into AD&D, I expanded with AD&Disms, but my game was solidly founded in that "default setting" for years. I don't think I looked at another published setting until someone turned me on to Ravenloft years later.
The multiverse in Marvel makes sense because we've been exposed to it slowly expanding out from a single core reality. We grew from Iron Man to the Avengers, added cosmic, magical, and time travel to stretch the setting farther, and then after Endgame, we finally introduced concepts like Variants, alternate timelines, multiverse magic, etc. Imagine if the first thing you were exposed to the MCU was Captain America First Avenger, Guardians of the Galaxy, Dr Strange, and Loki. You'd be lost to see how those elements fit together. It really helps if you start with Iron Man and work your way up through the concept of super-heroes by following the main through line (the Avengers) to ground the audience and give them a jumping off point.
The default setting (be it Mystara, Faerun, Oerth, or just the generic assumptions of the core rules) creates that jumping off point. It sets an expectation and understanding of what D&D is like so that when Spelljammer throws space at us or Dark Sun or Eberron take D&D in wildly different directions, we can say "woah, that's a cool way to do it" knowing what the default way is. Breaking everything down to its base chunks and creating Greyloft may sound cool to us, but I assure you it's a lot to take in when you're still figuring out the basics. You need a bland, boring starter zone to teach the default assumptions before you move on to the weird, esoteric stuff.
Now, I'm not against either of your products. A Worldbuilder Guidebook is a great tool for worldbuilding (the 2e one is still one of my favorite books). and a Grand Almanac of D&D Worlds is a product I've been asking for since 2014. But I still think you need a Faerun to act as a starting point for the people who are buying Starter Boxes or PHBs for the first time. Somewhere where the default assumptions of the game aren't challenged.
Think of it like this:
Beginner: Generic D&D setting (Faerun, Mystara, etc). Hews close to the Core assumptions
Intermediate: Specialized settings (Ravenloft, Theros, Dark Sun, Eberron) settings that change default assumptions about the game
Advanced: Worldbuilder's Guidebook to create your own setting.
