I've been thinking that a World of Greyhawk where the Savage Tide failed might be a cool place to adventure. I know Greyhawk is no longer something Paizo can work on, but I figure these fora can still be a place of fan discussion.
If we stay with the campaign arc's number of shadow pearls, I think they were only about 70 - considering the chaotic organization behind the Savage Tide, that would hardly be enough to cause more than a wave of terror. But imagine a genuinely successful Shadow Pearl blowout - with most large cities blown away. The situation is in many ways similar to the postapocalyptic games that bloomed around 1985. Most larger cities have been "nuked", but important people got away (save DCs are modest). The area of destruction is not huge (only a mile in radius each); the outlying areas of large cities would be spared, only to be savagely attacked by transformed inner city citizens - many still somewhat recognizable. Many cities would be "zombie towns" full of the transformed and those infected. Others will have curtained-of badlands where all the monsters live. Small bands of infected would prowl outlying areas, attack outlying settlements and provide a general menace. The largest cities, like Greyhawk, Dyvers, and Rel Astra, would probably survive because of their powerful guardians and large size. Rural states, like Nyrond, Furyondy, and all hinterland areas, would hardly be affected at all. The emerging city culture of Greyhawk would be setback, but not destroyed. Trade would grind to a halt with so many hubs of communication disrupted, but would soon resume at a much reduced scale.
Recovery could take decades - or maybe just years, depending on resources and heroes to lead. It would be a points of light setting initially, but with a known map and known objectives. Nations would come in to "succor" wounded cites, no doubt re-igniting old political conflicts like the independence of Dyvers. And once the old safe places are saved, the larger world looms.
Monsters in the hinterlands would not be affected at all, and would start raiding the weakened civilized areas. Ius' state seems largely rural, but would probably be disrupted by the loss of key marshaling fortresses. Also, the infected in this area would be particularity vicious because of the high concentrations of military force there. The Great Kingdom would be largely intact but politically beheaded; it is a rural economy, but all the political power is in urban areas. A great opportunity for an insurgent campaign in either of these places.
And finally, there is old monkey-boy himself and his minions, greatly strengthened and with a new unity. The chaos would engender apocalyptic cults, many demon-worshippers. The cults of various demon lords would be warring against each other. Powers like Iggliw that opposed the new king of demons will be in hiding and might have to be sought out for their knowledge. There will be a civil war among the succubi, with a weakened Malcaenet rivaled by the likes of Red Shroud and Shami-Amourae.
Overall, it will be a very chaotic world, and adventurers thrive on chaos.
Certain places could be... "infested". Savage Fever could have taken hold in more than just the creatures, but the actual life of the world. It's an agent of chaos, over time it's bound to mutate. Imagine swamps teeming with Savage Mist, forests where it's seeped into the trees, mountains where the very earth is infected. Change the infection method (injury, ingestion, spores, simple exposure), change the DC as it mutates, change the template it provides.
Anything that travels through "plague zones" may be prone to contracting Savage Fever, and eventually prone to contracting various different strains of Savage Fever. Cities may begin to more strongly regulate travelers, perhaps making them stay in quarantine for long times on end until they're sure they're not infected.
Rifts in reality, gaping portals into the Abyss, will probably become much more common as Big D and his minions take a stronger hold on the Prime. As a result the Baatezu are having to step up their pace in the Blood War, and the Good Guys are having to try and keep up... further hampered by the fact that it's a lot harder for mortals to be Good when the world's as bad as this.
I'd say look at Ravenloft and Dark Sun for more inspiration: the former has all your creep factor and general weirdness, and the latter is already post-apocalyptic in its own right and might toss in a few other good ideas.
This happened in my own campaign. Greyhawk underwent TEOTWAWKI and became a Points of light setting. Their 'zombie apocalypse' was worse than Faerun's spell-plague... basically what you'd see in most of the zombie apocalypse-type movies (especially plausible since savage creatures are more cunning and quick than standard zombies).
This was enough for the gods to keep the world quarantined, further retarding progress as trade with extra-planar travelers is hindered. After 100 years it is still a POL setting and planar travelers rarely hear about it.
"If the players fail, go ahead and blow up Vulcan" - High Level Campaigns Seminar
What really happens in a setting like this is that the monsters move out of the dungeons and move in as neighbors. The safe havens are less safe, the wilderness is more dangerous. Points of light!
I was looking at more of a "players patch up the setting" campaign that an infinite apocalypse, but making Greyhawk permanently be zombie-land works too. The problem with that is that so much of the original setting material is lost - I want Greyhawk to still be Greyhawk, just jolted. I generally have a lot of either light or at least hope of light in my settings. But the difference is one of degrees; the same setting material works for both. I just like a dynamic world where the PCs make a real difference, and that is easier in a world that is more messed up. I feel it is actually easier to be good in a world that is so strongly polarized.
That said, I liked the idea of plague locations and strains of savage fever.
My opinion is along the lines of Fyodor Dostoevsky: "The darker the night, the brighter the stars; The deeper the grief, the closer is God!" (Crime and Punishment) If you really want the actions of good players to shine, the worse off the setting is the more they'll stand out. My 2cp of course, you and your players' mileage may vary.
I was looking at more of a "players patch up the setting" campaign that an infinite apocalypse
Right. That's usually the expectation of most post-apoc or PoL settings. Things are supposed to get considerably better as time goes on (often at the PC's hand).
I seem to recall Dark-sun was like that, and even the original (1e) Greyhawk was more of an exploration/PoL type environment. Also, Faerun might've had a major spell-plaque, but it is still completely recognizable as Faerun (some areas even have almost no change)
Modern post-apoc shows like Jeremiah and Jericho also focused on the theme of bringing the world back into the light. They can provide some good inspiration.
If/when I restart my Greyhawk world, it would be after (or just as) a cure for the Savage tide infection was found. PC's would have ample opportunity to 'clean house' and re-establish communication/trade.
Things are supposed to get considerably better as time goes on (often at the PC's hand).
I seem to recall Dark-sun was like that.
Many people had a problem with this - Dark Sun was pushed as the original hellhole, but pretty soon became a campaign of world revolution.
Independent development like this are better in that regard - its much easier to discard what you don't like for a loose set of ideas than from a full setting. At the same time, you can be inspired by others' ideas.