More expansive and in-depth "Upgrading" system where you can upgrade weapons and armor.

An easy example would be in Monster Hunter, there were a few clear cut paths you could take to get the [item] you've been wanting. If we had a wide variety of upgradeable items that started from like r1 gear, that would foster some immersion. Additionally, more enhanced items that are of the same or very close rank to the base item, but that provide variations statistics, to allow crafters more choices for weapons, and combat classes more ways to tune how they want to use the weapon.
For example, enhancing a brass spear with one of the different gems changes the damage type to add 20% of the equivalent elemental damage type.

Inscribing your crafts

It could be as simple as doing a "touch up" by chiseling your name in, or taking a few ranks in "label making" to put your name on a glass of orange juice you just made! Many possibilities with this, and a lot of personalization to boot!. I would like to see this ability added as a crafter quest -- perhaps even getting an inscribe ability specific to each craft, perhaps as a follow up rank 40 crafter quest.

Behests for crafters
What if there were crafter-oriented behests? Could make these happen at the hamlets, perhaps, where:
Blacksmiths would have to repair ballistae, armorers repair the portcullis, carpenters repair the catwalks and drawbridge, alchemists prepare the boiling oil, etc, etc. This would be especially nice if hamlets are used for large-scale PvE with "hamlet defense" events, where the results of the crafter behests determine the strength of the NPC defenses when the combat classes later have the actual combat defense.

Group-crafting
For starters, allow that those crafts with multiple requirements to be performed by more than one crafter. Say a recipe is armorcraft 45, weaver 35, leatherworker 35. All one crafter to represent each class. For some rare synths REQUIRE more than one crafter.
Group crafting could work with the existing system by dividing all of the synthesis results by the number of crafters, and taking round-robin turns selecting actions and using abilities. Final results would be determined as per a single craft. The effective 'difficulty' of the craft would be based on the respective abilities of the crafters involved, for the profession chosen. If one craft is missing, the initiator's secondary craft skill is used.
(This idea is very rough, and likely needs feedback to improve it)

Gauntlet mode - a.k.a "Destroy that generator"
Allow crafters to participate in combat by addition of monster generators in dungeons. Monster generators cannot be destroyed by DoW/M, only by the appropriate crafter. Destruction of monster generators has an associated difficulty. Attempting to destroy a monster generator generates aggro, but being attacked in this situation does not automatically end the 'synthesis'. Your teammates will need to work to protect you and kill the monsters being generated while the crafter destroys the generator.