One thing that is rather noticeable about all the followers that craft something is how bad of value they get when initially being upgraded:
Upgrading them from level 2-9 and level 11-19 does seemingly nothing for almost all of them.
Quite a few of them have higher value at level 1 than they do at level 19, only becoming better once hitting level 20 for epic chance.
Ore cost increases with leveling, making the level 2-9 progression and 11-19 progression actively make them less efficient than they were at level 1 or 10 respectively.
These factors make it so around half the followers in the game actively lose value when doing their initial leveling.
One of the simplest fixes to this problem would be to make every follower that crafts something be like the Locksmith, where every level increases the chance for a double craft by 1%. That way a level 9 crafter would at least have a 9% double craft chance benefit rather than actively being worse than the level 1 variant.
The more complicated fix would be to give every crafter some form of constantly active passive that goes up every single level. Currently, Eveline is the only follower that both crafts something and has a passive that goes up every level. This concept with different mechanics could be applied to every crafter giving them some form of passive that goes up every level while still having their standard craft.
Some potential crafter passive ideas include:
Armorer: Gives % armor stat bonus per level
Artisan: Gives % resistance stat bonus per level
Mage: Gives spell damage reduction per level
Weaponsmith: Gives % power per level
Locksmith: A 5th mark category when completing dungeons with drop based on Locksmith level
The Runesmith: Increases upgrading material received from salvaging per level
Stable Master: Decreases pet rest time per level
Hunter: Increases chance of finding flux from chests per level