Rule 2: Find the Single/Multi-Class Balance
This rule has a number of parts. The first of these is detailed on the Wizards website, so I won’t go into it too much. Briefly, the idea is that when designing a class, it is important to make sure that every level gives a character something. My gloss on this is that a player should always look forward to levelling up, and that levelling up by continuing in ones own class should be just as exciting as levelling up by multiclassing.
My rule of thumb: If, at any point in a 20 class progression, it is clearly optimal to take a single level of Fighter instead of the next level in your class, there is a problem. (I use fighter for two reasons: (1) the first level of fighter is pretty good, and (2) the second level of fighter is near-identical to the first, so the problem will not likely go away in one level). One way of handling this that I find unsatisfying is to punish multiclassing. Gamers often forget that the objective is fun as opposed to the enforcement of rules (or even tropes). If I want to encourage single-class play I’d rather make it fun to play a single-class than make it un-fun to play multiple classes.
Do I want to encourage single-class play? Sure. I want to encourage all sorts of play. Why design a 20-level class if no one would take all 20 levels? On the other hand, multiclassed characters are just as valid as single classed characters. How many iconic characters in heroic fantasy fiction began as rogues or barbarians or wizard’s apprentices and moved on to something else? One of the things that bothers me about most d20 classes is that they seem to explicitly discourage multiclassing. Some of them (Monk and Paladin) all but prohibit it. Others (all spellcasting classes) put forward an opportunity cost that tends to be prohibitive.
While I want to make a class that will be fun and worthwhile to play for 20 levels, I also want to make one that doesn’t need to be. When making a class, think of the sorts of characters who might dabble in it… and think of the classes that a character of the class being created might dabble in. Build in some nifty features that will make such dabbling worthwhile. If I’m building a sneaky class, I might add in a feature that works well with sneak attack or or trap sense. If I’m building a divine class, I should be aware that characters taking levels in it might well have (or gain) cleric or paladin abilities. In thinking through the ramifications of this, I should avoid the gut reaction to nerf certain combos (No! That Cha bonus to Fort and Will saves doesn’t stack with Divine Grace!), and instead build the class so that its abilities complement those of other classes without becoming overpowered (Instead of a Cha bonus to Fort and Will saves, perhaps make it the Mettle ability – usable a number of times per day equal to your Cha – or Wis – bonus).
Tying this back in to Rule 1, as long as a character has 10 levels in the class I create, they should be fully recognizable as highly competent members of that class, even if they have multiclassed.