The official D20 core rules have 11 core classes and 5 NPC classes. There are many more in supplements, including additional 20-level "standard classes" and variants of the core classes. Each of the 20-level classes is designed to provide a full career. Many embody archetypes like the barbarian, wizard, rogue, ranger or monk that have deep roots in fantastic genres, or others like the paladin or cleric that perhaps the Dungeons & Dragons game has helped to create. Players have an accessible menu of choices they can make at character creation that will largely map out their character’s growth to the highest levels in the game.
These may be good things if that is what you want. However, I have a few problems with this official class roster:
It bugs me that, however many classes there are (and there are a lot), they still only provide a limited option set. In particular, features unique to one class are always tied to that one class. Surely warriors of many kinds from many backgrounds could experience battle rage - why do the rules only provide for this to be a feature of fast-moving, medium-armored, wilderness-based warriors who cannot read? Why not armoured knights or urban brawlers? Surely there could be many kinds of dilettante scholar with a wide knowledge gleaned from literature and legend - why do the rules only offer this ability to performing spellcasters with a bit of weapons training and a sideline in stealth? I want more flexibility, more mix-and-match.
It also hampers me as a GM that, within the official rules, I can't create NPCs who are as good at non-combat things (such as spellcasting or skills) as high-level PCs, but can't fight. A 10th-level Wizard is a pretty respectable combatant, likely superior to an ordinary city guard even without magic, and a 10th-level Rogue or Cleric much more so. I want to populate my world with non-warrior characters with little or no combat training - not just peasants and artisans, but wizards, priests, scholars, thieves and other experts able to play significant NPC parts. There is the Expert class, but since that has fewer skill points per level than the Rogue, it is not good enough. For spellcasters, there isn't even that.
I also find it unsatisfying that levels are not equal. Levels in NPC classes are worth very little. In contrast, a PC's very first level (or that of an elite, core-class NPC) has a bumper package of points and features. Even after the revisions of 3.5, subsequent first levels in other classes may provide bundles of proficiencies or core class features that bring an exceptional boost to the build. Higher levels in spellcasting classes bring major power gains, both compared to lower levels in the same classes and compared to even high levels in non-magical classes. And some spellcasters don't even have to forego combat abilities. I'd much prefer to know that two characters of equal level are of equal power, without having to look carefully at what class or classes those levels are in and exactly what abilities they carry.
The d20 approach to classes has huge potential that wasn't fulfilled in the official rules. I don't know if there was a conservative faction in the design team that wanted to stick with the old ways from AD&D, if there was a general agreement that most players wanted to choose class just once at character creation and from a menu of obvious fantasy archetypes, or what, but the outcome was that the core rulebooks never really embraced the potential of d20 to get away from the mentality that you have one class for your whole career and therefore that the game needs a different class for every career path. Pairs of classes persisted that were relatively minor variants on each other (eg cleric-druid, fighter-barbarian, wizard-sorcerer), or trios of two different classes with one essentially intermediate between them (eg cleric-paladin-fighter, druid-ranger-fighter). The NPCs presented in the core rulebooks and many supplements were single-classed and often pretty generic. And most of the advice to GMs and players about how character types worked together or fitted into the campaign world seemed to be predicated on single-classing and again sometimes on fairly generic choices within the classes.
I'm experimenting with a very different approach - 4 or so core experience classes aimed at replacing all of the official core and NPC clases, and perhaps more.
Because I'm playtesting rules for mundane things before attempting rules for magic, I'm initially playtesting just two experience classes: Expert and Fighter. I do anticipate introducing spellcasting classes in due course (probably two, possibly more, and probably all more or less built on the same pattern but with different details such as whether they use arcane or divine magic, and the detail of the spell repertoire).
I've constructed my classes based on the following principles.
This is right at the heart of character (and creature) building for me. I assume that characters will normally be multi-classed, probably from the beginning of their careers. I have designed classes not to be complete career paths in themselves, but to provide building blocks for multi-class characters. Obviously, therefore, I am not using any advancement penalty for multi-classing.
This discussion is primarily of what I call experience classes and the official rules just call classes - the levels you add in the course of your career. But I should note here that levels in mind and body classes (which the official rules call creature Hit Dice) work just the same as levels in experience classes for multiclassing purposes. This means that standard PCs add their bonuses and features from experience levels to those from mind and body levels, rather as in the official rules creatures with two or more creature HD and also with class levels add their bonuses and features together. For fuller discussion see my separate explanation of mind and body levels.
The flip-side of this is that monsters routinely have experience levels too; this is a major way for them to get skills, combat bonuses and feats. Adding experience levels is not a rarity for exceptional individuals of intelligent monster types; it is a normal part of creature building.
Hand-in-glove with multiclassing is that I have made each class grant features and bonuses of only one broad kind. So my Fighter provides only fighting bonuses and abilities - no skills or magic at all. My Expert provides only non-magical non-combat abilities - no fighting ability or magic at all. And the spellcasting classes will provide only magic - no fighting ability and no skills.
Purity enables me to get my non-combatants. I can build a civilian character with spellcaster and/or Expert levels but with very few Fighter levels. PCs and other adventurers would typically multiclass in Fighter levels to get a combat spellcaster or a backstabbing rogue.
This is already a feature of the offical 3rd-edition rules, but I wish to emphasise it more still. Feats, skills, spells and other choices of specifics within the broad class framework enable you to design the characters and creatures you want. I'm experimentally moving many things that are officially compulsory features of classes, like weapon proficiency, armour proficiency and most special abilities, fully into the realm of feats, skills or spells. This makes my classes much more customisable and should enable me to offer ways to rebuild most of the official classes.
Now, this is a matter for playtesting and improvement, but I'm aiming to make every level of equal power. Each class is to be of equal power with other classes, and each level in the class progression is an equal step in power increase. I'll aim to balance spellcasters with other classes better and give them a straighter power progression, and I'm not having any weaker classes for NPCs. If you want NPCs to be less powerful than PCs of similar age or status, you can just make them lower-level.
I'm therefore prepared to abandon the idea that PCs "start at first level". I'm not proposing to massively increase the power of starter PCs, maybe not to increase it at all overall, but I am proposing to acknowledge all the starter-character and first-class-level freebies by splitting them out into multiple levels. I mean things like getting all your weapon and armour proficiencies up front, maximum points on your first hit die, quadruple skill points for first level, a starting feat (often more than one), a larger number of spells per day and spells known than you will gain in any future one-level jump, and all the neat class features that come in the first lines of the level tables. These will all be spread over a number of levels, and starting PCs will begin with enough levels to gain most (perhaps all) of them..