Evidence or wishful thinking?
If I may indulge in a little of both...
We've been speaking of "Midway" as a PHB 2 type product; or a Big Book of Crunch. WotC doesn't produce those anymore. They produce hybrid products that tell stories along with giving DM and PC info. Volo's Guide is a great example, 1/3rd lore, 1/3rd PC stuff (races), 1/3rd DM stuff (monsters). Even the latest module, a series of unconnected adventures of D&D's past, has been couched in a framing device of "stories heard in Faerun's most famous bar".
Perhaps Midway is a Guide to the D&D Multiverse, with Sigil/Planescape as the framing device?
WotC has a few worlds that deserve some love, but maybe wouldn't warrant their own campaign guide. I could easily see a couple pages discussing Oerth, Athas, Eberron, Krynn, and Sigil itself (some general lore, a map, etc). Using Sigil as a framing device, you can have a planeswalker recall what he knows of each place since he visited them, creating an "in universe" description. Then, you add some needed crunch to run these worlds (Kender, draconians, artificers, mystics, warforged, shifters, changlings, kalashtar, etc) as your "crunch" portion.
Advantages:
* Good intro to the worlds beyond Faerun without taking the focus off the Realms
* Much needed crunch to run said worlds
* Might lead into opening the settings to DMsGuild
Disadvantages:
* Most of the UA stuff hasn't been very setting specific; not sure how you fit the new ranger or most of the new subclasses into world-specific desgin
* the multiversal setting would limit its use in AL play (unless the AL doesn't mind kender and warforged in Faerun).
* The only evidence for this is artificer (and to a lesser extent, mystic).