2 Types of PrC's
To me there are two types of PrC's: setting specific and non-setting specific.
There are those PrC's like Radiant Servant of Pelor, Red Wizard, or Iaijutsu Master that are clearly setting specific - you might adapt them elsewhere but they were designed with a specific organization, world, or religion in mind. I'd say that the majority of PrC's are like this.
However, there are other PrC's that I feel are appropriate anywhere. I must qualify this that these other PrC's are still limited, but not in the same way that the above category is. For example, where Red Wizards are only present on Toril (or originally came from there), there are certain PrC's that are more specific to a concept than a specific setting. For example, any world that has both fighters and mages has eldrich knights available. I may tweak the PrC ever so slightly to represent flavor but that is usually purely description - the mechancis would stay the same. Another example, any world that has mounted knights as a concept, will have the cavalry charger PrC. Another example, any world that has dwarves that are traditionally mountain dwelling folk will have dwarven defenders; any world that has elven "fighter/mages" will have bladesingers, etc.
I have created a homebrew world that is pretty much an iceball (world was wrecked by energy harnessed from the solar system's sun a few thousand years ago and consequently it is a lot colder on the planet now so the temperature outside is almost entirely arctic - you wouldn't believe how exhilerated I as to find out Frostburn was coming out). It is primarily a dwarven run world with most of the other races dying off. There are a handful of uncivilized human and orc tribes that inhabit the surfact, but most of what goes in the world is subterranean an controlled by the dwarves. As such, you are not going to find halfling outriders around. But you will find most of your traditional dwarven PrC's as well as a few homebrew creations. In fact, if you are any of the handful of races that are not dwarves, you can't even gain access to some of the core classes - there simply are no wizards on the surface of Grakin.
Interesting little tidbit about that world: platinum, gold, and silver are not considered valuable whatsoever. Instead, there are equal proportions (in terms of rarity) of adamentium an mithril. They are far more common on this world and thus are used instead of platinum and gold (respectively) with cold iron taking the place of silver. Copper still exists and because of the dwarven innovation of telecommunication (laid down insulated copper wiring between clans) they basically have an elaborate phone line system. Sorry I am hijacking the thread here, but I really liked this little tidbit: on Grakin, I have these little cricket like insects that are scarab like in apperance. The dwarves found out that they will sing if they are sparked in any way. Well there is another insect called a lightning bug (but it actually lets loose a tiny little spark). So when you want to make a call, you touch the wire with the lightning bug which causes a spark at the other end of the wire that triggers the cricket into chirping. Every call is routed through an operator's station and the operator will connect two wires so that you then can talk over the wires (its like when you were a kid and put the ends of a copper wire into separate styrofoam cups). I think I have got the science right and it makes sense to me regardless.
Ok - hijack is done.
