Lanefan
Victoria Rules
Unless there's mechanics in place that allow a character to pick up (an) additional class(es) during its played career.There are not a lot of other ways (that I'm familiar with) that produce satisfying results.
There's 2e style multiclassing which is only available at character creation.
Good. Anything that works against milestones or fiat-levelling is just fine by me.f you remove that restriction it either becomes absurdly good or unusable, depending on the experience system it's coupled to. (To say nothing of the fact that this type of progression basically requires XP, and won't really work with something like milestones.)

Otherwise, IME it's neither absurdly good nor unusable provided a few classes - Monk, Paladin, Bard - are not allowed to multi. And I say this as someone who generally dislikes multi-classing.
That was 1e, but yes, it's a terrible system2e style dual classing is a wonky and ludicrous approach even before examining it from a balance perspective.
And if the game doesn't use or have feats this one's a non-starter.There's feat style multiclassing like we saw in 4e, which worked okay in 4e (but not phenomenally) but wouldn't work in a system where you aren't regularly getting feats because having to wait four levels to multiclass is fundamentally unsatisfying. Many campaigns will end in that time.
The problems with 3e multiclassing, repeated in 5e, are:Is there some perfect multiclassing solution you're aware of that I'm overlooking here?
While I agree that 3e style multiclassing isn't perfect, I don't agree that it can't be improved to work well. We've even had some suggestions to that end in this very thread.
--- the additive aspect; where a 4th-4th character is treated as an 8th rather than a 4th in each of two independent classes
--- the all-or-nothing approcah to levelling where this level you advance one class and next level you advance the other, in that it doesn't reflect how the character is being played in the fiction. If I'm a fighter-thief, in theory I'm using the skills of both classes while in the field and my xp should reflect that; the classes should advance independently (i.e. each have their own xp track) and level up whenever they happen to level up. Further, they shouldn't have to level up together - if I play my F-T mostly as a thief then I should be able to, with DM approval, assign more xp to thief - say, on a 75-25 ratio - and have it advance faster than fighter. (I also like and use variable-by-class progression tables, but that's another story)