I own nearly every pre-3E Greyhawk book/boxed set, and some 3E ones as well.
You don't need special rules to run or play GH. It's just D&D. (Or Rolemaster. Or Burning Wheel. Or . . ..)
If anyone wants to play GH, just buy a copy of the boxed set from DM's Guild and start playing!
Psionics is its own thing - though a sorcerer might be used to approximate it.
Athasian bards, without spellcasting, can be done as assassin rogues. The actual 5e bard is a spellcasting class - I don't think we're going to see it rebuilt as a non-caster.
Defiling magic can presumably be done the same as in the 2nd ed books, or else using the 4e variant. In 2nd ed AD&D, the rules for the effects of defiling can be ported wholesale to 5e (I just reread them to make sure). Being a defiler rather than a preserver gives, in effect, a bonus to earned XP (approx +40%, meaning that quite a bit of the time from 2nd through name level the defiler will be a level ahead of the preserver). In 5e you could just do the same thing, or if in the hard cold light of 30 years of D&D experience that seems too broken or ad hoc you could do something else - eg +1 to save DCs and attack rolls for the defiler.
I'm not meaning to cast aspersions on your imagined book. Just trying to point out that so much of this material is already able to be used in 5e with minimal fuss and effort.