So one of the things I really like about Pathfinder is Paizo's theme of stepping away from prestige classes and focusing on archetypes. If you look at 3.5 Edition, you find a dozen or two classes, and about a thousand prestige classes, probably more. Now, don't get me wrong, I like the specialization of prestige classes, I think they're neat. But I like archetypes a lot more. What an archetype does is take a class, remove a few features from it, and add others in to give it a theme. Basically it is a low power, level one prestige class. As you advance in your class, you don't get the class' normal features, you get the archetype features. So for instance, if you have a 1st level Fighter and want him to use the Unbreakable archetype (an archetype focusing on indomitable will and unstoppability) you lose proficiency with tower shields and your bonus first level combat feat, instead, you gain Endurance and Die Hard as bonus feats. At 2nd level, you lose Bravery and gain Unflinching, which gives you a +1 Will save bonus against mind affecting effects that increases to +5 over the course of twenty levels. just like Bravery would normally do. I feel that archetypes are superior to prestige classes because they allow you to tweak your class in a sometimes big way, but don't force you to step out of that class and lose advancement with things like sneak attack or spells per day or your animal companion's power level. Granted, they are not as powerful as stuff like the Assassin or Shadowdancer. I think this gives you more flexibility as a player as well and keeps the number of prestige classes down so the ones that are in the game remain... prestigious I guess.
One thing I've wanted to do with the campaign book is add in The Known World prestige classes, and I still may do that. I was going to add in ones for Witch Hunters, Blackcoats, Brasscoats, Serethnem Riflemen, and some sort of Monk/Psion one for The Old Empire.
Now Brasscoats and the Monk/Psion idea definitely need to be prestige classes due to the advanced abilities I'd want to give them. But Blackcoats, Kusseth's trench warfare and stealth specialists make much more sense as Rogue or Ranger archetypes. They keep the core concept of the class, but add specialty with explosives and dragonspitters and pitch black combat to it. Witch Hunters make a lot more sense as an archetype of Psychic Warrior focusing on combatting magic. Sereth Riflemen make more sense as Sereth Gunslingers focusing on utilizing the Sereth long rifle. I need to look at the campaign book, I'm not sure if I have the appropriate stats for that firearm in there at the moment.
I guess the archetype idea is easier to implement than prestige classes as well. I mean, all I really need to do is look at the archetypes already in the various books and make some tweaks to them to fit them in with the concepts behind Witch Hunters and Blackcoats and whatnot. Shouldn't be too hard to do. I am always leery of creating completely new stuff built out of nothing in the game, I have only a vague understanding of balance and assume there is at least some level of playtesting that occurs with every feat, class, and archetype in the Pathfinder books. Or at the very least some guidelines on what some of that stuff does. I'd love to see a feat/class/archetype book similar in style to the Advanced Race Guide that breaks everything down into a point system with guidelines on power levels and creation of feats and classes and such.
I dunno, if I do end up making any of the above in the near future here, I'll post it here on the blog, probably.