D20 Mechanic: Racial and Regional Classes
While any race can become any core class, there are cultural and racial leanings that should also be represented. If halfling gunslingers wielding gunblades are common - at least, more common than halfling wizards or rogues - then there should be mechanics present to help reflect and engender this sort of thing happening at the table.
The important thing to remember, I think, is that these aren't restrictions. We're not saying that an orc can't become a wizard. What the presence of racial and regional classes do, however, is reinforce the idea that while orc wizards can happen, it is less likely to happen among the normal orc population, because of their racial and cultural leanings.
In this section, we'll cover three types of classes - racial paragon classes, racial prestige classes, and regional advanced classes.
Racial Paragon Classes
Every race has a racial paragon class. This is a three-level class that has no prerequisites aside from being a member of the appropriate race.
Racial paragon classes focus on a race's natural abilities, improving and slightly expanding upon them.
A character who finishes out their racial paragon class is exempt from all the prerequisites of their race's racial prestige classes - that is, they are treated as though they meet all the prerequisites, even if they do not.
Racial Prestige Classes
Every race has at least one racial prestige class. This is a ten-level prestige class that, under normal circumstances, can be taken upon reaching 5th or 6th character level. As noted under racial paragon classes, however, a character that has finished out their racial paragon class is exempt from all prerequisites for all their racial prestige classes, meaning that they can theoretically be taken as early as 4th character level.
Racial prestige classes focus on the areas that a particular race prefers to study and exemplifies. While in some cases this can be just an extension of the race's normal abilities - such as the orc predator - some of them are a fusion of two or more core classes that a race particularly focuses on culturally - such as the halfling regulator - while others focus solely on a single area, with a spin on it unique to the race - such as the gremlin saboteur.
Regional Advanced Classes
While not necessarily every region will have an advanced class, most will. This is a five-level advanced class that, under normal circumstances, can be taken upon reaching 3rd or 4th character level.
Regional classes are similar in concept to racial prestige classes, but instead are based upon a given geographical region's tendencies and preferences, rather than a single race's focus. As such, it is entirely possible that there will be a Coldlands Territory advanced class, focusing on technology and gun-use, but with a Coldlands Territory feel.