In the AMA, Mearls hinted pretty heavily that we'd see some sort of branching out to different settings over the course of 2018. This gave me an idea for a thing I'd very much like to see in a setting-focused book, assuming there's enough space: new backgrounds.
The backgrounds in the PHB are, by necessity, fairly generic. But take a setting like Eberron for example - there are many cultures there that would like some kind of in-game treatment. You have the Argonessen dragon-worshiping barbarians, the Ghaashk'kala tribes who guard the Labyrinth surrounding the Demon Wastes, the Valenar elves, dragon-marked houses (either as a single background or specific ones - or maybe do it like the Far Traveler in Sword Coast Adventurer's Guide, and have one background with a few variants), and so on. This would add a lot of flavor without too much in the way of mechanical weight.
Sure, you could handle a lot of them via the Outlander background, but I'd rather have different backgrounds for different places.
The backgrounds in the PHB are, by necessity, fairly generic. But take a setting like Eberron for example - there are many cultures there that would like some kind of in-game treatment. You have the Argonessen dragon-worshiping barbarians, the Ghaashk'kala tribes who guard the Labyrinth surrounding the Demon Wastes, the Valenar elves, dragon-marked houses (either as a single background or specific ones - or maybe do it like the Far Traveler in Sword Coast Adventurer's Guide, and have one background with a few variants), and so on. This would add a lot of flavor without too much in the way of mechanical weight.
Sure, you could handle a lot of them via the Outlander background, but I'd rather have different backgrounds for different places.