The above, from Wikipedia, is my class and multiclassing system. It comes from Talysman's post on the three-role class system, which recognizes that there are three basic archetypes inherent in the early game, which also comport with archetypes present in fiction - the fighter, the wizard, the trickster. The fighter approaches problems directly, with force or toughness, the wizard, using magic, and the trickster, indirectly, but without magic - applying her own cleverness and skill.
Therefore, I have:
The Warrior: Receives training in two different weapons per level, and reduces Death and Dismemberment Table rolls by one, per level.
The Mystic: Receives one extra spell die per level, and can craft magic items using a system I have yet to satisfactorily determine.
The Spy: Receives training in climbing or swimming, per level. Receives training in stealth, per level. Begins with literacy in native language, and can use the Lore system to gain knowledge of additional languages or scripts, or otherwise rumors and gossip.
Each of these is one of the "primary" colors - say the Warrior is red, the Mystic is blue, and the Spy is green. Overlap represents a "dual-class" - which I'm constructing to be a fully-fledged class, since that is much clearer and easier to conceptualize than a hyphenated Franken-class. So: between the Warrior and the Mystic is the Warlock, who gains weapon training at the cost of crafting ability. Between the Warrior and the Spy is the Assassin, who retains the weapon use, stealth, and terrain abilities of both classes. And between the Mystic and Spy is the Trickster, which is a stealthy spellcaster.
Obviously, there's no reason to mix between any of the derived classes - there's absolutely no reason to be 1/4 this and 1/3 that, or to pursue any of the more esoteric shades of green. There is, of course, the central mix of all primary colors - the polymath, who is stealthy, gains some weapon abilities, and is a spellcaster.
Incomplete, as usual. But I think I'm going to do away with separated hit die progressions, attack-bonus tables, and definitely with weapon and armor restrictions. That way, each class is as concise as possible, and imposes much less upon your character's stats and abilities.