Expanding the atlas to other Golden Age rpg worlds
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).
Yep, Penumbra and all of the hundreds of Spelljammer worlds would be included on the overview map of the Material Plane; and some of them would be given at least a small sketch of a world map.
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). They'd be retrofitted for the Great Wheel.
If we're going to go that far, then Troll World (1975), Bunnies & Burrows (1976), Wilderlands of High Fantasy (1976), The Misty Isles (1977), Arduin (1977), Glorantha (1978), Archaeron
(1979; Chivalry & Sorcery), Bleakwood (1979; Adventures in Fantasy), Kèthîra (Hârn; 1983), Palladium World (1983), Pendragon's Arthurian Britain (1985), Talislanta (1987), Mythic Europe (1987; Ars Magica), Shadow World (1987), Space: 1889 (1988), and Earthdawn (1993) would be great old-school additions to the Atlas of the D&D Multiverse.
There's nothing inherently stopping the D&D Brand Team from reaching out to whichever companies hold these rights to see if they could make an agreement and bring all these worlds fully into the D&D Multiverse.
Ideally the agreement would stipulate that beyond the initial inclusion in the Atlas, plus the option of making one worldbook, WotC would have a perpetual right to briefly mention that world as being part of the D&D Multiverse.