Seperate armorer & blacksmith rank 20/30/etc quests
My suggestion is to make the armorer and blacksmith leves separate.
As you know you get a crafting class leve at rank 20, 30 and 36. This leve involves creating an item and in the end it rewards you with gil, and more importantly guild marks.
However, for some reason the armor and blacksmith guild leve are put together. The quest itself is different for both classes, but you can only choose to do 1. So if you do the rank 20 blacksmith, you can no longer do the rank 20 armorer one. It's possible to alternate, like r20 blacksmith, r30 armorer and r36 blacksmith, but that doesn't make much of a difference.
Because of this, anyone who has both armorer and blacksmith loses a lot of guild marks compared to other classes. To be exact:
2000 + 3000 + 3600 = 8600 guild marks
20000 + 30000 + 36000 = 86000 gil
Now, I don't care as much about the gil. It's nice that's true, but the guild marks are 10x worse.
Now, I can't really come up with a good reason for the developers to make this choice. I know there are some similarities between armorer and blacksmiths, but not more than other closes, and not enough to justify this decision in my opinion.