Devs usually base class design on the idea that you have to have defensive classes, damage classes and support classes. For the most part that's pretty much how any class-based RPGs work.
I can't imagine how you could make a class-based crafting RPG. It'd be more like a job simulator lol. Final Fantasy XIV does have several crafting/gathering classes outside of their combat classes, though.
Obviously the most important parts of class design are making sure they are unique and balanced. In a lot of class-based RPGs I see, some of the classes are basically just the exact same thing with a different look. The playstyles are exactly the same but they wield different weapons and wear different armor. Their skills/abilities are just renamed versions of the same thing. It's very common. Also very common is RPGs that have the one class with the stat/skill combo that can be exploited to own everything in the game with ease.
