Here's the thing. I have very, very limited time and I don't even had time to prepare everything for my adventures, let alone the rest of the game world. If you do, great. Now, I'm very good at improvisation, so it doesn't bother me to make something up on the fly. However, many DMs are not good at improvisation and need those details figured out in advance.
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Having an updated version of Greyhawk with those details filled in by WotC would allow those DMs to run Greyhawk.
You can't just expect those sorts of DMs to just "make a decision" when it comes up. Creativity is not the same as making a ruling on a rule.
The DM not having those details can work for some groups, but other groups are going to be unsatisfied with a bunch of clueless NPCs.
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A new Greyhawk setting would do well for a myriad of reasons, even if you wouldn't buy it because you can make stuff up.
The issue that
@3catcircus raised id
not a lack of detail. It is about conflicting details.
A GM who has only one sourcebook isn't going to be bothered by those conflicts - s/he won't even no about them - and so they aren't a problem. Any GM who has bought multiple sourcebooks and has noticed the conflict is already committed enough that I'm sure s/he can spare the extra 15 seconds to make a decision should it be required.
As far as running GH, the point I have repeatedly made is that
any 5e GM who wants to run GH can do so. All s/he needs to do is buy a copy of the boxed set of DM's Guild for about $10. That has evertyhting s/he needs: maps, descriptions, history, etc. The information about the levels of the leaders is all usable, because every class mentioned is a class or subclass in 5e. The only mechanical detail that can't just be used in 5e is the information about the gods and clerics - but 5e has its own well-developed rules for specialised priests (ie cleric subclaasses) and so my hypothetical GM will be able to use those in lieu.
If people are declining to run GH because they won't do so until WotC tells them which 5e cleric subclass they should use for each GH god, well, I would think that's a pretty sad state of affairs.
GH is almost a special case. It exists in a state of veneration unlike Dark Sun, or some of the other settings that come up as prime for reissue. The problem WotC faces is that hardcore Greyhawk players don't really need the setting book. They are probably already playing in the setting and may or may not be playing 5e. The veneration for the setting also derives from it's Ur status as the first D&D setting, written and developed by the man himself. So between the enormously diehard fans and the setting's status, I think that for GH the internet shouting would be magnified no matter what the 5e book looked like. On top of that GH doesn't really have anything to offer new players, or at least anything new. Don't get me wrong, I love Greyhawk, but there's nothing different about it, or at least IMO different enough that the rules or crunch would be enough to sell the book to people not interested in the setting, and the setting itself perhaps isn't different enough to attract enough new players. Anyway, my point is that I think GH presents some additional difficulties for WotC and as a result may not be as high in the design queue as people would like.
GH is one of the few published setting I've ever used, and over the years I've run close to two decades worth of games in it. I own nearly everything published for it pre-3E, and some of the 3E material also. That probably makes me a hardcore GH player.
I don't care what WotC publish in future in respect of the setting, though I probably won't buy it given that I would be unlikely to use it. Most recently I've been using GH for a Burning Wheel game, using the amps and the basic lore, but certainly not worrying about what level ranger the king of Keoland might be. My existing material - which tells me that Hardby is a tributary city to Greyhawk and is ruled by a magic-wielding Gynarch; and that the Bright Desert is a place where (to quote one of my BW players who was hoping to round some up) "Suel tribesmen are as common as fleas on a dog" - is eminently suitable for these purposes.
If WotC
does decide to publish a GH setting book I am very confident it won't be in order to authoritatively resolve the question of the king of Keoland's class and level. Nor will it be to tell people that their battle-axe wielding bear barbarians should come from the north and their open hand monks should come from the Scarlet Brotherhood: anyone can work that out!
It will be because they think there is a significant market among new players for whatever new mechanics they've written up, and for the setting in which they;re locating them. It seems unlikely to me that that would be anything which is
distinctively Greyhawk or aimed particularly at existing GH players.