One of the design priorities of the early editions was crafting a human-dominant world. But I’ve often questioned the game-design choices made to accomplish this goal.
The supposed strength of humanity in old-school D&D — its racial ability, if you will — was its flexibility. Human characters, alone among the other racial choices, could advance to unlimited levels in any class. Other races were restricted in both the classes they could join, and the levels to which they could advance in those classes.
But this led to interesting contradictions. The one that always stood out to me was that elves, supposed masters of magic and the wild places, could only advance to about 9th – 11th level in the magic-user class, and couldn’t be druids in most early versions of the game. And humans, supposedly the most flexible of the races, could not multi-class at all. Instead, they had to use a clunky “dual-classing” rule that required them to abandon one class completely, and focus exclusively on a new class.
So, the most flexible race in the game could only be one class at at time, while the supposedly less flexible demihuman races could multi-class freely, albeit in racially-restricted combinations.
This never made any sense to me. My house rule always struck me as an easy fix, and served me well in older campaigns.
The rule is simple, assuming a human-dominant fantasy world is what you want: humans can freely multi-class in any combination to unlimited levels (barring other restrictions like alignment and ability scores). Demihumans can multi-class, too, but only in their racially-defined combinations, and only up to listed level limits. However, I’d allow elves unlimited advancement in the magic-user class, to reflect their arcane heritage.