I voted ‘other’.
The idea of a minor class with only a few levels that a character can multiclass into, seems solid.
Certain character concepts seem bigger than a feat, but smaller than a 20-level class. For example, race features as levels, getting a ‘pet’, maybe wildshape. or so on seem worthwhile as a class that has upto 5 or ten levels.
These mini classes could even work by tier. Some classes might be available at levels 1 to 4. Other mini classes might only become available at higher levels, like 5 to 10, or whatever.
In any case, a particular concept that more than one class might be good at, might work better as a mini class, rather than rewriting a separate subclass for each class, each one doing the same thing.
In hindsight, ‘healer’ could have been a miniclass. Then the Cleric, Bard, Druid, Paladin, Sorcerer and others could have taken some levels in it, if they wanted their character concept to access the very same healing spells.