I haven’t had time to crack the previews I’ve received of the Adventurer Conqueror King Player’s Companion, but conversation with the Adventurer Conqueror King or Die! group I play with on Saturdays suggests this idea isn’t there.
Right now all PCs get a general proficiency at first level, fifth level, ninth level, and thirteenth level, plus bonus proficiencies for high Intelligence. Each PC also gets a class proficiency at first level, then another every few levels (depending on class), at a rate faster than they gain general proficiency slots.
I don’t remember seeing anything about multiclassing characters. AD&D it was either too hard (human dual-classing) or too powerful (demihuman multiclassing). D&D 3.x had a good idea, but it failed in execution because of how it interacted with spell casting classes — or rather, it probably would have worked well because it brought casters back in line with other classes, but the perception of ‘heavy nerfing’ won that argument. Echelon doesn’t even have classes and has, I think, the best option for mixed-archetype characters… but we’re not talking about Echelon here.
D&D 4e, though… what if we took an idea from D&D 4e?
In Adventurer Conqueror King, there are a number of proficiencies. Each class grants access to certain proficiencies. These proficiencies may be taken with class proficiency slots… and as far as I can see, only with class proficiency slots, and the class proficiency slots may only be spent on those proficiencies. Similarly there is a group of general proficiencies that may be taken with general proficiency slots, and only with general proficiency slots. Some general proficiencies are also in class proficiency lists.
What if the rule was relaxed a bit? The rules for class proficiencies remains the same, but the general proficiency slots gained from class levels (or those from class levels and the bonus slots from high Intelligence, if you want to be even more generous) can be spent on any proficiencies the character wants.
In many cases ‘non-class, non-general’ proficiencies are useless. James Ironwall (fighter) would gain no real benefit from Black Lore of Zahar (well, the ‘control undead as a Chaotic cleric of one half his class level’ could be useful, sometimes, but having targets of his spells requiring Death saves take a -2 penalty to the saves isn’t so useful, nor is having his necromantic spells such as animate dead cast at two caster levels higher, and I’m not sure why someone with Int 7 would be doing necromantic research anyway). However, if he gets (druidic) religion or otherwise becomes closer to nature, Beast Friendship (identify plants and fauna on 11+, understand the moods of birds and beasts, gaining a +2 reaction roll when encounter normal animals, and being able to take animals as henchmen) very well could be a good thing.
I can imagine a rogue with a familiar (a fighter too, but given the primarily combat role he faces the potential sudden loss of half his total hit points is pretty limiting). I can see several other class proficiencies that could be applicable to other classes.
Of course, this opens the door to having a character spend ‘general’ proficiency slots on more class proficiencies. I’m not convinced this is a bad thing, though I do appreciate how the current rule forces characters to look both inside and outside their classes for the proficiencies. You might say general proficiency slots may be used for ‘general proficiencies, or proficiencies from classes other than your own’… or better yet ‘general proficiencies, or proficiencies not on your class list’ if you want to prevent picking a proficiency from another class list that also happens to be on your own.
Just an idea, I’d be interested in seeing what people think.
The default approach to multiclassing from the beginning – a fighter/magic-user chosen at creation, for example – is to use the class creation guides in the Player’s Companion to create the hybrid you want. I think your approach is well suited to a character who has leveled as a single class and finds that they want to diversify. The class limits should be seen as establishing genre – these are the things expected of a fighter, or a rock n’ roll song – not hard limits on what instruments you can use and still rock.