Gruagach elves for one. Scarlet Brotherhood monk, Knights of the Hart, Circle of Eight agent background, Druid circles built around the original druids from 1e. There is a LOT to do with Greyhawk in 5e that hasn't been done and yeah you don't need... Dragonborn. Tiefling's would be fine in Greyhawk. There are Greyhawk specific monsters like the Greyhawk Dragon to name one besides the aforementioned Gruagach elves (Valley Elves). Actual Explanation of the Greyhawk deities and not just a list of names and their domains maybe? sure some of the data is there but the material itself is negligible and barely skeletal. Greyhawk is not the generic fantasy people seem to think it is or want to force it into. It isn't just Forgotten Realms, nor is it Tolkien. It is more Lankhmar but definitely Vance and an ample helping of Burroughs. It's rougher, it's dirtier, it's less heroic. It's more down to earth while simultaneously more Cosmic and Moorecockian.
If anything Forgotten Realms, as much as I love it, is the more generic of the two but that is its nature by design as it was intended as a kitchen sink setting where you can find almost any sort of setting from Canadian wilderness to Conan like Thay and faux ancient Middle East.
Greyhawk isn't this B&W, we're big dang heroes setting, it is shades of grey. It isn't good vs evil, it's Law vs Chaos and fanaticism. Not quite Mork Borgian levels but man, it's a lot darker than people seem to want to give it credit for. Dorakaa alone is the stuff of nightmares and the Pomarj? Unlike Waterdeep is Greyhawk really a city you would want to raise children in? What about the Great Kingdom with Ivid the Undying? This is not a happy place. There is not a Cormyr with shining knights and an Arthurian king. There isn't a peaceful Dalelands that occasionally deals with Zhent spies because Elminster is a busy body. No the great wizards of Greyhawk aren't even plotting for good but to keep both good and evil, law and chaos, from gaining too much ground.
I've seen the excuse people don't want grey, they want B&W and heroes these days except we know now they don't. They want escapes and adventure but not always the good guy. People like having an edge and Greyhawk has an edge. It's a backstabbing thief in an alley and a barbarian warrior laughing as he smashes in your head and Emirikol laughs as he rides off with both their bags of gold.