Generic hybrid classes

 Last post, I came to the conclusion that the vast majority (if not all) D&D classes are, functionally, built around three primary approaches to adventuring: combat and physical prowess (fighter,) magic (magic-user,) and mundane non-combat skills (thief.) By hybridizing these three, you can adequately model almost any sort of character without creating a new class for every single variation on a theme. 

There are a few different ways we can go about hybridizing classes. One way is to make the new class fully functional in both of the basic classes, or nearly so, and making XP requirements much higher to compensate and/or applying a level limit. It's essentially a "best of both worlds" concept, applying the most advantageous aspects of each class, e.g. the higher hit die, the less restrictive weapon and armor strictures, the better combat, spell, and/or skill progression of the two base classes. Basic D&D does this with the Elf class. Except for the d6 hit die instead of d8, it has the full advantages of both the fighter and magic-user classes. This isn't my favorite method, however, primarily because, increased XP requirements aside, it's still much more powerful than either of the base classes, and even doubling XP requirements means characters generally only lag a single level behind their peers.

Instead, I prefer to use a sort of compromise paradigm. A fighter/magic-user, for instance, will be better than a pure magic-user at combat-related challenges, but not as good as a pure fighter, and likewise, can cast spells where a full fighter can't cast any, but not as proficiently as a full magic-user. The factors which must be considered for compromise are several, including hit dice, equipment restrictions, and combat, spell, and thief skill progression. Let's take a look at each element in turn.

Hit Dice can be a perfect splitting-the-difference if the base classes are two steps apart, as with the fighter and magic-user or fighter and thief classes. Just use a d6. The only other combination for basic D&D is magic-user/thief, whose base classes both use a d4. Again, easy.

Combat progression is likewise simple for the fighter/magic-user, which has the thief progression smack-dab in the middle. In the case of the other combinations, I would use the more advantageous table, i.e. fighter in the case of fighter/thief and thief in the case of magic-user/thief.

Spell progression is trickier. One way is to slow the rate of progression itself. For instance, the hybrid class casts spells as a pure magic-user of half level (or two-thirds if you want a little more oomph and don't mind the math); rounded up if you want it to have a spell at 1st level, and rounded down if you'd rather make them earn it at 2nd. The other is to use a restricted spell list, as the traditional D&D cleric does. I favor the first approach, since it doesn't require the creation of special spell lists. Of course, spell choice may be restricted due to cultural reasons. A fighter/magic-user "battlemage" might be barred from using healing and illusion spells, while a fighter/magic-user in a good-aligned church might be disallowed spells that cause direct damage.

Thief skills may be handled similarly. Either they function at some fraction of the hybrid class's true level, or a limited selection is used, such as four of the eight thief skills, either assigned by the DM or selected carte blanche by the player.

Weapon and armor restrictions should generally be the more favorable of the two, except when reason dictates otherwise. The most obvious example is restricting the fighter/thief combo to leather armor, since metal armor lends itself poorly indeed to any sort of stealth. Equipment restrictions can also be applied as cultural strictures without being tied directly to a particular class combination. For instance, a fighter/magic-user who belongs to a religious order may be forbidden to use edged weapons, while a fighter/thief who is a member of a guild of assassins may be required to use them.

For XP requirements, if the two base classes are relatively close together, use the higher of the two. A fighter/magic-user would use the magic-user XP progression table, while a fighter/thief would use the fighter table. If the two are far apart, as are the magic-user and thief, it may be better to split the difference and apply the fighter table, which lies somewhere in between.

Saving throws may pull from both classes, using the best number from each class for each category. A fighter/magic-user could use the fighter's saves vs. poison and dragon breath, while using the magic-user's against spells.

A final question that arises, given that there are various formulae that may be used to construct a hybrid class, is whether to create multiple hybrid classes based on applications of the variant formulae. Should you make a fighter/magic-user who advances in spell ability at half-level and one which advances at full level with a restricted list? That's ultimately up to the individual DM or designer, but I'd say no. The difference between the two is ultimately not significant enough to warrant the added complexity and the analysis paralysis that may afflict players faced with too many options. The same applies, I think, to fiddly bonuses and penalties commonly used to distinguish similar classes, e.g. a +2 to attack rolls with a specific weapon or against a category of enemies, an extra spell of a specific type, or a bonus to a category of thief skills. The impact of such adjustments is generally not worth the bother of keeping up with all the extra complications.