. A setting like Dragonlance or Dark Sun removes some common races but replaces them with unique options. A setting like Theros or Ravnica is attempting to emulate an extremely specific setting that wasn't designed with D&D in mind.
And right there is why there cannot be agreement.
“Every other setting, from Drsgonlance to Theros, may be different. but not Greyhawk. Because REASONS.”
No thank you. You have the Realms as the default in 5e.
The absolute last thing that any Greyhawk fan wants is for your solution. Just publish the Flanaess as a new continent for the Realms to swallow, republish WG7 as canon, retcon Bigby and Zagig as Elminster and punch every Greyhawk fan in the face.