This may sound surprising, given the plethora of classes I posted in the last couple weeks. But those classes were meant more as examples of customizing existing classes. I have a certain system, as I'm sure everyone noticed.
First, there's the experience/hit dice model. The four core classes provide this:
|Base Class||Base Experience||Hit Dice|
|Support (Thief)||x1/2 + 1/8||+1/2|
|Hybrid (Cleric)||x1/2 +1/4||x3/4|
|Combat+Turning||as Cleric||as Cleric|
|Support+Turning||as Cleric||as Magic-User|
|Magic+Turning||as Magic-User||as Magic-User|
|Combat+Support||as Magic-User||as Cleric|
I've written these to show how the other classes are modifications of the Fighter class. A Thief isn't as effective in combat as a Fighter, so Thieves only get half a hit die when they advance and only need half as much experience, plus a bonus for their unique abilities. Magic-Users likewise only get half a hit die, but magic is so useful, they need just as much xp as a Fighter, plus a fourth as much. Clerics are half-way between Fighter and Magic-User, effectively making them three-quarters of a Fighter for both experience and hit dice.
The combos represent mixing part of one class with magical powers based on Turn Undead. The "Combat+Turning" combo represents the Beast-Master or fighter-ly Bard: effective as a Fighter, but able to use a small list of themed supernatural powers repeatedly. As you can see, it's just a standard Cleric as far as XP or HD go. "Support+Turning" represents the thief-ly Bard, who doesn't have all the thief abilities, but has powers bordering on the clerical. In either case, we drop or reduce one of the abilities of the base class (Fighter or Thief) and change the theme of what the turning mechanic affects (charming animals: Beast-Master; charming people: Bard; charming weather spirits: Weather-Worker.)
"Magic+Turning" represents specialist magicians like the Conjurer, the Mesmerist, or the Necromancer, who use the Turn Undead mechanic, but can also research spells to use in scrolls (but can't memorize spells.) We could upgrade some of the other combos to this, to give spell scrolls to Weather-Workers (turning them into Weather-Wizards or Storm-Lords, whatever you feel like calling them.) Magic+Turning classes are customized by changing the theme of the turning mechanic.
I've added a hypothetical combo that I haven't discussed yet: a Fighter with a couple Thief-like support abilities added. They use M-U experience and Cleric hit dice. It's probably better to use the changing class rules for this, though.
Aside from changing themes for the "X+Turning" combos, we can also change the theme or target of support abilities in the Thief class, to make Tinkerers, Dwarf Smiths, or Dwarf Miners, for example. Similarly, we can change the spell list or spell prep of the Magic-User: substitute Cleric spells for Magic-User spells to get White Magicians and Black Magicians, substitute cheap potion creation for memorization to get Alchemists, swap familiars for spell books to turn Alchemists into Witches. And anything you can do to the base class can be done to the combos as well.