There's some possibility, here. It would have to be handled right, though, to ensure it didn't turn into a mockery or be done in an overly specific way.
The game is already vaguely pseudo-Medieval, with obvious influences from romanticized images of both feudal knights and Greco-Roman high culture. Because of that, I don't think having a book devoted to either Carolingian or Hellenic setting would be a good plan. There just isn't enough to separate it from the core. What little there is would make for an extraordinary Dragon article or series.
Oriental Adventures didn't try to make it so you could play the game in mythic China or mythic Japan. It blended a bunch of cool concepts from Asian myth into something that was nearly as pseudo-Asian as the core D&D was pseudo-European. Ditto for what I know of al-Quadim. There wasn't a very high effort made towards cultural realism, just fun settings. You could easily do the same thing with African lore -- keeping in mind that you're going to end up with a "what if" where Ethiopian technology kept pace with the Mediterranean for an extra couple centuries, while keeping the animistic "Dark Continent" magic from pulp adventures.
Personally, I think I hit on my preference, early on: make better use of Dragon. At what point did Dragon become the players' magazine and Dungeon the DM magazine? I thought Dungeon was all about adventures, maps, and dungeons. In the heyday of the 1980s, we got Dragon articles about "The Ecology of the Remorhaz", "Introducing the Half-Ogre", the Duelist class, critical hits tables, mechanics for pain, and "Be Aware, Take Care". There were many articles delving into the mentality of the elves, dwarves, and other races without presenting any new stats (though, they sometimes explained the psychology/physiology behind the stats that were there). Sure, there were new spells, magic items, and other mechanics (and new feats would have felt right at home) but there was a ton of meat that didn't have numbers directly associated with it. That would make a much better outlet for some of these ideas than a new book.
In truth, I think Dragon would make a much better outlet for a lot of classes and races than a PHB2 or PHB3 would. I don't want to see the developers feel obligated to include myconids or shardpeople as "core" just because it's time to put out the annual PHB.