This. A thousand times this.
A while back, an idea (vaguely based on a WotC tweet) about a Worlds of D&D book was floated; 320 pages, with one setting per chapter. Each chapter would have a general overview of the setting, a few rules/options, a monster or two, and a "where to find more" section.
1.) Forgotten Realms (General info on Sword Coast/Neverwinter area, Sundering changes)
2.) Greyhawk (General Flanaess info)
3.) Eberron (Five nations info, Xen'drik, warforged, artificer, and shifters)
4.) Dark Sun (general info, elemental clerics, defilers, athasian subraces)
5.) Ravenloft (power checks, curses, domain rules)
6.) Planescape (Sigil, bariaur and githzerai races, and factions)
7.) Dragonlance (knighthoods, draconians, wizard orders, and kender subrace)
8.) Birthright (dominon rules, bloodlines, and mass battle)
9.) Other Worlds (a bit on Mystara, Kara Tur, Zakarha, and Nentir Vale. No real rules info here).
As a whistle wetter, it'd be a good intro to other areas and give most conversion projects a starting point. However, I've learned not to hold my breath.
We've got the fluff for all of those settings covered, several times over, with past editions. I'd rather see the 5e take on the crunch for these settings.
I'd also really like to see a robust document on converting material from BECMI/1e/2e/3e/Pathfinder.