When the game first came out, there was a discussion about multi-classing to a fair amount. These are the conclusions I've reached, based on those discussions.
In general, there are three types of multi-classing that I've found:
- Dipping - you just want to steal another mechanic from another class to use for your main direction, popularized in 3e.
- Dual Classing - you're mixing two classes into one. Wizard/Cleric hybrid, for example, or the Paladin/Warlock, both classics.
- Career Changes - a story-driven transformation where you renounce one profession for another - the reformed thief/warlock being most obvious, the retired warrior another, etc.
Multiclassing, as per the alternate rules, really only supports Dipping. Its not intended to have duel classed options.
If you want a mixed class, the only real option is to go with a subclass. Want a mystic theurge type? Make a subclass that mixes cleric and wizard spell lists. Hells, we have that with the new sorcerer's favored soul test anyways. This is really the only option available, imho. Choose a base class, then alter a subclass so that it imports a few of the abilites ofthe other so that it feels duel-classy. Its not the same as swapping out abilities, but I've seen some homebrew that successfully change them around so that it functions similarly enough.
Career changes have no rules for them, and have been handled many ways, depending on the situation at hand. Sometimes, like a fallen paladin, we can get away with just swapping a subclass. Sometimes, we reset a party member to level 1 on a new class. Or convert them to a different class at the same level. And more. There's lots of individual context that comes up that having hard rules doesn't work for everyone.