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Seb-wejem
This is another reason why I would have liked subclass abilities to be gained at the same levels for every class, to aid in turning a class into a subclass that can be taken by anyone. That way, the fighter could take a barbarian subclass and gain barbarian abilities like rage, fast movement, and unarmoured defence (along with whatever higher level barbarian abilities may have been available). They wouldn't necessarily gain every barbarian ability, but would gain just enough to add some barbarian flavour to the primary class.