I've never considered Greyhawk, Forgotten Realms, or Dragonlance as unique to be their own settings. Certainly you can just shoehorn the best parts of Greyhawk into a continent of Forgotten Realms. Have the Free City be a rival to Waterdeep or something. Let your characters see both places - why not?
Wait ... were you a product manager of TSR in the 90s?
Now I am become ABEIR-TORIL, the destroyer of CAMPAIGN SETTINGS!