so we got one thing, and we need some races, subclasses or a whole class to get it sellable?As others have noted, it's not about what you can't do - if you want people interested who don't have nostalgia, the setting needs to let them do something well that they can't do well with the existing tools. Ideally, it should be both thematically appropriate for the setting as established and something that can easily be ported over to other settings. For best results, it should also be a player option to maximize sales.
Stronghold/leadership rules could work, but most 'setting books' have included new races and/or subclasses.
Those are good points. As a DM I care more about DM options (low magic, stongholds, followers, exploration rules, etc.), but I don't think they are likely to sell a setting book that only restricts player options. Is there something from Greyhawk lore that could be added that would be new to 5e?This is a good point. To extend the analogy with Greyhawk, Theros removed 7 races from the PHB (which are extremely common in fantasy properties) and replaced them with 5 races that aren’t commonly seen in tabletop games.
I would suggest that this, plus a couple of other elements from the book (emphasis on piety, gods as major figures in the setting and the Greek mythic inspiration), contribute to making Theros feel different from generic high fantasy.
But to get back to Greyhawk, if we were to limit races (not replace races with different uncommon races) AND also restrict classes, I’m not sure that Greyhawk would have the same success as Theros.
Those are good points. As a DM I care more about DM options (low magic, stongholds, followers, exploration rules, etc.), but I don't think they are likely to sell a setting book that only restricts player options. Is there something from Greyhawk lore that could be added that would be new to 5e?
Well, I think we have solved all of the problems then. I see no reason a Greyhawk setting couldn't offer something unique and new with options for DMs and Players. Now we just need to convince WotC!Sure! There are a number of things that could be added.
Obviously you would want to avoid more specific rules for variant humans, but depending on where we are going with race/ancestry, we could include rules for playable xvarts (an EGG favorite for some reason), Half-Ogres (variant from Dragon), Valley Elves (gag me with a spoon), Duergar (UA), Gray & Wild Elves (Grugach), and Tallefellows. You'd probably different rules for the cambion/Tieflings for Iuz and the Great Kingdom.
You could have additional subclasses by concentrating on regional specialties- the Scarlet Brotherhood with variants of Monk, Thief, and Assassin is obvious, but there are many others.
That's just a start- I'm sure others would add in.
Because F-you Hickman and Weis. That's why.
WOTC outright owns the IP. But DL fandom still consider DragonLance to be Hickman and Weis's baby.
That's why Butcher bailed on WOTC when he found out Hickman and Weis had nothing to do with the reboot he was supposed to do.
A Song of Ice and Fire is a human only setting with low magic power, extremely low magic frequency, and extremely low magic versatility. However it is a very popular setting due to the thick political drama, layers of romance, displays of action, suspense, mystery, and tragedy, and gripping lore.
Greyhawk 2.0 cant be just "It's D&D but only 2 of you can be a caster or nonhuman. It'll totally be fun guys". Because that puts all the burden on fun on hope.

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