As regards setting, I have some suggestions (with the assumption you want to have a Modern, not SF setting, ie not a setting with interstellar star travel):
1) Create one setting. Every DM can create his own setting. Some want to roleplay in a favourite book, but you won't get IP rights, so this is immaterial. If anyone else is interested in a professional setting, it is because they want to have it ready to play and don't have to pay too much attention to it. In other words, if a GM wants some specific setting, he will create it himself. Accordingly I suggest a single setting, but broad enough to use for different purposes and different types of campaigns.
2) Since you will not be able to use a licenced setting, the best way to design one is to start with the kinds of games you can roleplay in it. There are two main activities in a modern RPG game - investigation and combat. You want investigation to take place in a fairly well-known setting - with cities, libraries etc, and usually with police. Alternatively, it can take place in a wilderness, with scarce population - but it will tend to quickly turn into combat.
As for combat, it should take place in some spot when the players won't be afraid of police and won't be able to call for help.
Optimally, there should be also exploration - strange vistas, exotic temples, traps etc. And treasure, including powerful weapons you cannot buy in a shop.
Finally, you need a way for the PCs to gain money without a steady job.
3) It suggests the world should be similar but not identical to our own. Moreover, it should be divided into two zones - one more or less civilised, and the second wholly out of control.
4) The solution - you need a mixture of Stalker and Warhammer Dark Future, and perhaps TORG.
http://en.wikipedia.org/wiki/Stalker_(film)
http://en.wikipedia.org/wiki/Roadside_Picnic
http://en.wikipedia.org/wiki/Dark_Future
http://en.wikipedia.org/wiki/Torg
In Stalker novel by Strugatski brothers, Earth have been in a short contact with an alien reality (UFO landing or interdimenishional contact - it is not known). As the result there appeared destroyed Zones, filled with deadly natural hazards and incomprehensible alien technology, some of it quite magical.
5) Short description of the proposed setting:
Alternative Earth, technology level a bit more advanced that modern, but more variable (you can get some SF artefacts if you pay enough, but day-to-day technology is obsolete from our point of view). States, rulers etc are called the same, but aren't identical (the President of USA is called Bush, but he is not the same person). The borders can be different, but it should be left open.
This Earth have been touched by an alien reality. As the result there appeared Zones, where reality works according to a bit different rules. You can find there monsters, deadly traps (reversed gravity, high gravity etc, acids, illusion, deadly artifacts) and wonders - some incredibly expensive. No official forces would enter a Zone, so inside you are on your own (cell-phones don't operate). Some Zones are quite extensive (there should be a big one in the Amazonian jungle, in Kongo etc), some small (there can be a mini-Zone which covers a few buildings in a city). They are used by criminals, would-be wizards etc. Some Lovecraftian horrors can dwell inside or come out and prey on a city, also vampires etc. PCs are by default free-lance specialists who search Zones for anything valuable (you can find there longevity potions, universal cures, and even artifacts granting wishes - and much more valuable things). They can be also secret agents or private detectives.
It is not commonly known, but some Zones can be used as a gateway to quite different realities. Those other planes are filled mostly with ruins, monsters and few crazed survivors, however.
As a result of that irruption, many states have fractured. Capital cities are mostly under control, but in the out-back there rule militias etc. Even in USA there are some completely lawless areas which are not a part of Zones. Middle East is in the grip of a much more terrible insurgency. The Egypt is controlled by it already, and it influences much of Africa.
Their leader is known only as Black Mahdi. It is rumoured that he has been killed and managed a resurrection. It is certain that his followers are sure that he can give them eternal life. As the result, he has many recruits from the West, some of them high military officials or scientists. They already managed to use a synthetized pox and advanced computer viruses against USA, and some desert states are nearly depopulated as the result (luckily, they didn't managed to deploy it on the coasts).
http://en.wikipedia.org/wiki/Storm_Worm
http://www.terrorisminfo.mipt.org/Dark-Winter.asp
http://www.lewrockwell.com/north/north573.html
http://usacac.army.mil/CAC/milreview/English/MayJun07/Hammes.pdf