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 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, showing exactly where every D&D world and plane ever mentioned, in any edition, are 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 names.
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.