4e did different from the design proposal in the original post.
4e was ‘Advanced D&D’ in the sense of continually updating, but it lacked ‘Setting D&D’. It made the mistake of trying to combine all of the *unique* settings into one mash-up homogeneous supersetting.
First of all, D&D has always combined all settings into it's 'Multiverse' conceit. But, 4e's rules, constantly growing & being 'updated' as they were for the first two years, did work for all, what? 3-4 settings that got books in its short run.
5e's rules obviously won't work smoothly for some other-than-FR settings, but they're not even meant to work smoothly for any given campaign, they're /meant/ to have significant table rules and variants ('modules' whatever, I'm old, a module is a pre-packaged adventure, alternate rules are variants). When we get adventures or supplements for other settings ('when' - we already got CoS, Ravenloft isn't exactly 'in' FR), we'll get variants to go with them, probably choices of variants.
What 4e should have done is let the Forgotten Realms setting remain true to the Forgotten Realms setting.
I can't say I'd be broken up about any Apocalyptic shake-up of FR, including it's annihilation. ::shrug::
Psionics of Dark Sun. You mention the mystic class, but also possible are psionic subclasses for other classes, and psionic feats: should these become part of the Forgotten Realms setting or become part of the ‘chapter’ on Dark Sun?
Psionics has been in FR before, though, hasn't it?
Before Eberron, FR was the kitchen-sink setting.
And likewise, let Dark Sun and Eberron and so on be completely separate settings that had nothing to do with each other.
Eberron was a 'kitchen sink' setting (as of 3.5), everything in D&D had a place in it, so that's hardly screaming for it's special snowflakeness, but it does mean there's a lot of stuff in Eberron that's not in 5e, yet - and that Eberron wouldn't be a bad place to introduce stuff, either...
... the conceit of Dark Sun or Dragonlance or whatever being 'cut off' from other D&D settings was more about the fuzziness of the thinking back then, the simple idea that you go from one DM to another, you're playing in a different campaign, didn't really sink in for everyone, some players felt their old (utterly broken Monty Haul) character should be able to just Plane Shift into your campaign from whatever exercise in insanity birthed it. Likewise, too many D&Ders took "in the rule book" to mean "in the multiverse, and thus likely to pop up on any world," DMs could assert the right to dictate what was in their campaigns, and we often did - and so did some official settings, to retain thematic purity - you can't just say 'no clerics' when a Cleric can Gate into the setting from FR any time (actually, you can, you just felt like you needed an extra layer of explanation why not back in the day - you don't, you're the DM, you never did). I think you could safely add material for any number of settings without DMs feeling obliged to let it into their particular campaigns.