
Here's a piece of Abeir, the continent of Laerakond, which swapped places with Maztica during the Spellplague. Maybe they've swapped back for the Sundering?
If you're going for obscure, there is also Penumbra - the artifical discworld of the illithid race (couldn't find an existing map via google).
Also, Tekumel - the Empire of the Petal Throne


Though it's going beyond the original concept of an atlas which brings together all the TSR/WotC worlds of D&D, it would be a bonus if WotC reached out to the key "Golden Age" rpg settings from other game companies and incorporated those worlds into the D&D Multiverse (or rather, a parallel version of those worlds, so that that the "non-D&D" version of the world is considered to still exist in another reality).

Not quite - Tekumel was printed by TSR itself in 1975, though Mr. Barker must have kept the copyright to be able to publish it elsewhere - including Tekumel would be akin to including Lankhmar, Kalamar & Hyperborea in the atlas.
The others you mention, I don't believe ever had an "official" TSR publication.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.