[MENTION=4937]Out of curiosity, what would you use as your base classes.
Ok, so I could definately condense down further than I do, but if I did I'd be losing mechanical variaty and a bit of flavor variaty that I find important to my game world.
1) Fighter - This is the core class of fantasy gaming, the one you have to get right. In my game, it includes soldiers, knights, samurii, martial monks, warlords, and really every who makes a profession of mastering combat and the art of war. One of my goals for the fighter has been to try to make it reasonable to make any of the six attributes your key stat and build a fighter around that.
2) Wizard - The wizard is in my opinion a strange beast and in many ways the modern fantasy wizard is a pure creation of D&D. It doesn't have a lot of mythic tradition, and at some level I'm tempted to do away with it but on the other hand the notion of the wizard as scholar is just so important to how we see the wizard now that I just feel I have to have it. No other class is as deep in terms of support you can provide for it. Few other classes really resonate as well with the modern mind.
3) Rogue - The third archetypal fantasy character, and the class I've played the most as a player, so I don't really feel I can do away with it. The 3e version was well done, and all that it really needs is a minor boost past 10th level, a bit more skill points, and a bit more ability to be proactive with its skills.
4) Shaman - This is based off the Green Ronin version of the class, and generally replaces Druid, Witch, Adept, Witch Doctor, and to a certain extent 'magic user' in my game. By dropping Druid, I could expand into its territory far harder than GR could, essentially gobbling up its spell list. I cut pretty hard against the grain in see the divine spell casters - particularly the shaman but also the cleric - as having a much stronger mythic tradition than what we call 'wizards'. In fact, what most ancient cultures called wizards, we'd call shamans or priests. The fact is, people aren't stupid. They knew that people couldn't do magic. But they believed that there were higher powers that certain people might make bargains with or gain the favor of or find ways to leverage and command. That is a 'wizard' by most historical usages. It's only really when we get to the 19th century that we begin to replace in our minds the notion of 'wizard' with the notion of scientist.
5) Champion - This is the first pure homebrew class on the list, which gets in my game by way of Holy Warrior and Unholy Warrior which were Green Ronin's somewhat hesitant but still quite inspired attempts to fix the Paladin class and make it generic. GR's take had a variaty of problems, partly because they wanted to make the class backward compatible with the 3.0 Paladin, and partly because of some poor editting - especially in the Unholy Warrior source book. But freed from those constraints I think I've made a pretty great class that more or less can fill the shoes of every 'annointed', 'appointed', 'ordained' or 'committed' archetype regardless of what they are representing. These are ideas made flesh. To a certain extent, this is arguably droppable, as you could probably get here by multiclassing two of the other classes, but notably it is not droppable unless all nine of the other 'core' classes are present, at which point you might as well include it. You can't drop it to cut down to 3 base classes without losing things.
Arguably, here we could stop. And as I indicated, I think you could drop the wizard if you really needed to pair down. We have the three core archetypes, and Champion is flexible enough that in many ways it can serve as every other class concept. However, I really want to go further, particularly into diversification of generic fighter into things that could be made into feat trees, but to me fill more solid and easily balanced as separate classes as well as exploration of flavor not easily capturable in the mechanics of Wizard or Shaman.
6) Explorer: This is my generic hero class. Also a homebrew, it's been around in some form since 1e with it's genesis being originally a broadening of the concept of the 'Mariner' class from Dragon Magazine. It got ported in to 3e by me initially as an NPC class that was used by pirates, bucanneers, teamsters, gypsyies, and generic adventurers. Gradually though, its power level rose as I gave it more stuff to the point that it became competitive as a PC class. It has become a great class for Duelists and other lightly armored swashbuckling types that depend on skill and motion as much as weapon process. They have a bonus feat list built around fencing, two weapon fighting, skillfulness and speed. In it's present form, it's been a very successful class at low level but worry about its high level viability. Still, it's definately higher tier than RAW 3.0 fighter. Explorer steals a lot of the Ranger stuff that wasn't about animism or huntingl, and competes with a rogue as skill monkey trading the sneak attack for full BAB progression, more hit points, and other features. Very popular as a dip class so far in my games, which was intended. It's a great way to bridge classes or get more well rounded.
7) Hunter: This takes the other half of the Ranger's martial stuff and goes deep with it, become a focused killer - especially with ranged weapons. This is the class of regular hunters, bounty hunters, executioners, asssasins, demon slayers, undead hunters, and yes - with a bit of multiclassing with something like Shaman - traditional rangers. One of the most important advantages that the class has is that they can make critical hits on any favored enemy - even those normally immune to criticals. As such there is huge and intended synergy with Rogue, to the extent that if you meet an assassin in my game its pretty much gauranteed that he is a multiclass Hunter-Rogue. Multi-classing with Explorer gets you a guide type character. In addition to ranged weapons, the Hunter schtick is massive criticals.
8) Fanatic: This is the class of the warrior who is powered not solely by skill at arms, but also by raw emotional energy. Built off the barbarian, but intended to also represent elite bodygaurds, dervishes, religious fanatics like the Jewish Nazarenes or Zealots, elite assault troops like the Aztec Jaguar warriors or modern US marines, psychotic maniacs, and yes raving spittle flinging Norse inspired beserkers. I borrowed a bit of Monte Cook's Oathbound flavor to help fill them out, dropped alignment requirements, and gave them a somewhat configurable package of skills representing their different traditions.
9) Sorcerer: I wasn't a big fan of this class when I first saw it, but it grew on me. The real insight for me was that even though it was just a variant Vancian spell-casting class, it had a flavor that wizard did not have. This was The Mutant. This was the meta-human. This was the person whose power came from within - both the superhero and the thing that ought not be. This is the class of Spider Man and of Wilber Whateley. This was the class of Professor X, Wolverine, Nightcrawler, and Elastagirl. You just had to customize your spells (and expectations) accordingly. Once I got that, it was an easy hop to giving the class a list of feat trees that give you the ability to mutate into something obviously inhuman. Tons of 3rd edition prestige classes and templates can be captured in this idea.
10) Cleric: This is the class of the true holy man. This is the class of the servant of the gods. The reason I have both Shaman and Cleric is the same that I have both Shaman and Wizard (and to a lesser extent Wizard and Sorcerer). There is a very different flavor here. In my game world, there is a sincere and bitter theological and philosophical rivilry between those that look at the world like Shamans - something to trick, intimidate, befriend, coerse, command, but in general get to do your bidding - and the Clerics who believe that they exist to serve the universe and not the other way around. Note that this is not entirely an alignment argument, though it sounds simplisticly on law versus chaos on the surface. This isn't really about society versus the individual, but rather what your role in it is and on a larger scale what is the purpose of humanity. Both sides in my opinon have very good reasons for believing as they do. This tension is almost the third moral axis in my campaign, as relevant as good vs. evil or law vs. chaos. Is humanities place in the universe near the top, or near the bottom and if so, "Why?"
Ok, there you could stop for a generic campaign world IMO. Anything you add after this will make your world much less generic. In my opinion, just about any concept you could make up, any base class or PrC you could examine from a published book, could be carefully constructed from the above and a feat tree or two. So what falls after here are actually very ungeneric:
11) Bard: For the longest time I considered dropping this class. The only reason I keep it is that the concept of singing your spells relates nicely with a major concept in my campaign world called the Iconoclasm (not to be confused with this worlds Iconoclasm), and the fall of the 'Art Mages' - a dead but far more potent magical tradition than currently exists in the world of the 'present'. Bards in my game represent a small surviving fragment of that knowledge. Interestingly, that means that they are inherently members of a secret society that most of the world would happily execute as practioners of divinely prohibited art.
12) Feyborn: A custom class that exists because there are no halflings or gnomes in my world. If you want to play a wee person, you have to go fey. This is a racial class for my three fey PC races, and its built around a jack of all trades that gets an expanding customizable list of spell-like abilities. So far no one has really explored it so I'm totally unsure on the balance, but I have seen it dipped in some highly amusing ways.
13) Akashic: A slightly buffed version of Monte Cook's Akashic just fit so perfectly into my existing game world that it was beautiful. Essentially another racial class, this time for a PC race called the Idreth, whose primary schtick is that they share a collective racial memory. They don't actually reincarnate (at least no more than any other race), but they do remember fragments of the lives of their ancestors. This class's flavor assumes that it is about practicing meditation so as to be able to recall more perfectly the knowledge of those former lives.
Coming down the pipe I'm working on what will probably be nearly the last class I introduce ever, the Paragon - a class based on having an epic destiny, luck, and divine favor.
Lastly, I still have a few NPC classes for when I don't want NPCs to shine so much. I don't have adepts or aristocrats and I almost never use warriors and may drop them, but Expert like Explorer before is growing in power and may eventually hit the point where it could be a PC class option. The other three NPC classes in current use are Brute, Scholar, and Commoner, but Scholar may be on the chopping block as its possible to get there now via Expert and certain trait selections. Currently, Explorer contains the keys to your 'Investigator' concept, but if I ever figure out how to move Expert up at least a tier, then it would take over that flavor. That would leave me with 13-15 PC base classes, and 2-4 NPC classes. It's possible that Expert will end up swallowing Brute at some point, which at that point would mean 15 base classes + commoner.