Parmandur
Book-Friend
Sorry I didn't phrase my post very well. What I meant is that even if this boxed set was a campaign setting, I'm not sure how much more incremental value it would add compared to the products currently released.
So let's say you're correct and it's a FR campaign box. The lore would have to be spread out over the entire world, otherwise it's just rehashing the stuff already released in the Sword Coast Adventurer's Guide. And if it's the entire world, they need to spread it thin to cover everything in Toril. Plus some of this info is already covered in Out of the Abyss and Tomb of Annihilation that cover areas beyond the Sword Coast.
Now they could still do a campaign box, there's enough remaining content (and they can repackage already released content), I just don't think it's very smart on their part to do this. More likely IMO, they're going to keep releasing new Adventure books with each one covering a different area of the world of Toril (like Al-Qadim or Kara-Tur).
If it's a new starter box, the choice to me is Greyhawk because it doesn't require new rules and can function as a baseline default setting, much like FR does. And at the same time they can release content for that world that is completely new for FR.
If it is truly a campaign box, I think it's Dark Sun or Eberron. Both require new rules for their worlds to actually work, plus the obligatory world lore would be completely new.
So, there are 7 items in the box set that the are floating there to test people's reactions:
1- Special Dice
2- "Poster map of the campaign setting"
3- "A campaign or adventures that capture the flavor of a D&D setting"
4- Mini-handouts (they call out spell cards as a possibility)
5- Character sheets
6- Basic rules
7- A campaign specific DM screen
Of those 7 items, WotC released ancillary products that hit 1-4 alongside Ravnica (Dice and "Maps & Miscellany").
Given what they have revealed about Target wanting board game style packaged D&D TTRPG material, making an Eberron box to compliment or stand in for the book, and sell in Target board game sections makes sense.