I'm hopeful this is a full on separate book (think like that Exodus sci-fi game WotC produced but didn't create) and not "just" a supplement, so it can adjust the 5.5e rules to be closer to old-school styles while using the current D&D "engine". No tieflings/aasimar/goliaths/dragonborn/orcs as playable races, for example, since that doesn't fit Greyhawk (no "weird" races, period, just standard things including Half-Elves and Half-Orcs). Maybe automatically use some of the optional rules like short rest being 1 day and long rest being 1 week, that kind of thing.
If you want to play a game in which there are no Goliath PCs, or no Dwarven Wizards, then why not jus tplay it? I'm not sure why you need WotC to give you permission, or direction.
Likewise with the rest rules. I mean (i) these have already been published and (ii) you seem to have already set them out in your post. If you think they're good rules, why not just use them?
our GREYHAWK 1E games generally never had Orc player characer, and Orcs were rarely seen freely in even the largest cities, most often than not prisoners.
Those playing Half-Orc were assumred to be of the 10% sufficiently non-orcish to pass for Human.
The PhB (p 17) is pretty clear about PC Half-Orcs:
it is assumed that player characters which are of half-orc race are within the superior lo%,
But the DMG is equally clear about the presence of a variety of (typically evil) humanoids in towns and cities: on p 191 a footnote to the City/Town Encounter Matrix notes that "If desired, 1 in 4 [ruffians] can be half-orc or of humanoid race (goblin, hobgoblin, kobold, orc)." This is consistent with the text from the GH setting that
@Voadam quoted.
He said they will absolutely be doing things like having inherently Chaotic Evil Orcs
In AD&D, Orcs are Lawful Evil by default.
They won't really win back old-school fans who aren't happy with the current edition/tone unless they scale back some of the rules too IMHO. More than anything else, the issue is that most old-school players think the rules themselves don't lend themselves to a grittier style game, which is why they gravitate towards rules that emulate B/X or AD&D. If they don't fix that, this project is DOA.
Nothing could kill any interest in this product more than this for me
OSR players have a new retro clone coming out every other day but it's never enough. Won't be happy until they AD&D 3rd edition is made with all the racial limits, class restrictions and level caps back in place.
There's no reason at all to think this project is DOA unless it restates 25+ year old rules for building PCs and NPCs. Anyone who wants those rules, and want to play a game set in GH and using those rules, can already do so.
The market for this product is current D&D players, plus those who are nostalgic for GH material. I don't think the GH nostalgia crowd is all that big a market; so, to avoid being DOA, this produce needs to be purchased by a reasonable number of that first cohort.