While searching for something completely unrelated, I came across this system by Arcana Creations for archetypal multiclassing. One might also call it class-and-a-half multiclassing, for reasons that will become clear. It was written for Castles & Crusades, but looks usable with any similar fantasy game. The basic idea is that a multiclassed character has a primary class and a secondary class, and the secondary class is reckoned at half the level of the primary class.
There are a few brilliant ideas here. For example:
- Least restrictive weapon list but most restrictive armor list
- Experience needed for advancement: primary class + (secondary class / 2)
- Most secondary class powers only accrue at second (i.e., 2/1) level
- Limit of 2 classes helps prevent multiclassing for optimization purposes
- Average hit dice with bias toward the lower (e.g., d6 and d8 = d6)
- Fighter/magic-user is different than magic-user/fighter
It’s not explicitly stated, but I would say attack progression was take best.
Though I prefer allowing all weapons (basing damage on hit dice) and armor (but making armor penalties more salient) no matter the class, I still think the “least restrictive weapon list but most restrictive armor list” is very elegant for those limiting weapon selection by class.
Since you only really get access to the secondary class powers at second level (as first level is 1/0), there are few immediate benefits, so motivation is less likely to be power gaming. For example, a fighter/magic-user does not get to prepare any spells until second level, though they would be able to use magic items or cast from scrolls as a magic-user immediately. A first level magic-user/fighter would be able to wield any weapon though.
One variation I might try would be to roll hit dice for both classes and then divide the total by two rather than adjusting the hit die itself since I like to re-roll hit dice every adventure.
Example: Fighter/thief at level 5. With my hit die variation (and based on Moldvay hit dice), HP would be equal to (5d8 + 5d4) / 2. Minimum XP required (numbers from B/X) = 16k + (9.6k / 2) = 20.8k. Functions as a second level thief. At level 6 will function as a third level thief.