If I was doing Greyhawk first thing I'd do is cut the number of nations by about two thirds. There's way too much repetition, the place is far too balkanized
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Relevance:
1. More female rulers and NPCs.
2. Environmentalism vs commercial development. Environmental disaster, perhaps represented by magic gone awry while attempting to control elemental forces such as giants or the Temple of Elemental Evil.
3. Economic decline, possibly due to the aforementioned disaster. Hits City of Greyhawk hard. Many made homeless.
4. A new, good-aligned ruler comes to power in the Great Kingdom. Ends war, forges diplomatic ties with good-aligned nations.
5. Minor wars still rage. With the Baklunish if you want to be super-obvious. Savage humanoids in their mountain strongholds would be another option. Or perhaps dwarves.
The world I recently made up for a session at the weekend had economic decline in a Roman/Holy Roman-style empire, and commercial exploitation of the environment as its big issues. Both were probably unconscious on my part.
<- Relevant
Let me jump on the badwagon and say that this is NOT the way to go to make GH relevant. This is the way to go to make it bland, and in many ways the fault I find with FR.
I could reserse this to make my own list:
Relevance:
1. More courtly intrigue and succession issues, largely ruled by custom.
2. Feudalism vs commercial development. Monetary economy vs. feudal economy..
3. Economic advancement creates social disorder. City of Greyhawk becomes so rich it disturbs many of the old powers.
4. A new lawful-aligned and competent ruler comes to power in the Great Kingdom. Tries to re-unify the country, bringing peace and "good" results, but not necessarily by good means. How do the players face this?
5. Minor wars still rage. With central authority weak, conflict is const both in border regions and between feuding nobles. War is as much within each culture as between different cultures.
In my personal GH campaign, I play up the cultural conflicts between the different humans - and how these are slowly merging into a "common" mercantile culture with strong points in cosmopolitan cities like Greyhawk, Ironforge, and Sasserine. I play up how, good or evil, there is a natural conflict between various interest groups - Feudal vs. Merchantilist, Republic (Perrenland) vs Monarchy (Furyondy) and so on. Sure, these are just backdrop elements, but that is what a campaign world is all about.
I must also agree that Golarion feels much like Grehawk II. It is very easy to steal elements of golarion and fit them into Greyhawk - some almost feel like they were originally written for greyhawk. Red Mantis Assassins => Scarlett Brotherhood (or a part of) - who conveniently worships a god of insects named Bralm.
I am considering stealing the nation of Andorian, the republic from Golarion, and placing it in the northern part of the old Great Kingdom - culturally it seems like a perfect fit. As my next campaign is set to play out in Ahlissa in the south of the old Great Kingdom, this would be quite relevant (but again backdrop).
Another long-ranging plot I've been thinking of is to have Dwarfs invent and mass-produce the Warforged - and instill them with many dwarf virtues (loyalty, thriftiness). This seems pretty close to how Warforged already are. Together these two races try to enforce "stability" in western Flaness - which inevitably leads to conflict with both giants and humans.
This is one thing I like about an officially "dead" setting - it is ALL MINE to play with. There will be no further official releases that I have to decide whether to use or not. Of course, this also means that the setting will disappear from the minds of the gaming audience at large.