I have to agree that creating an artifacts outside of a mood strikes me as odd. We already have a sub-artifact (the master work item) that has a sizable bonus above the next tier (exceptional).
Where I was going with my statements about ambushers et al is that currently we do get spammed with bone carvers and stonecrafters like the OP said. Finding some tangible way of making other skills go legendary that has the same power/effect of the current craft moods just seems like it should be there.
Even some of the current mood make little sense. My miner goes moody, makes a stone crafting item, and is now legendary? How about instead, he digs a tunnel 10-20 tiles in a random direction (even cutting through an engraved wall) and finds an ore vein not normally found in that rock type?
I thought the mythic plump helmet suggestion was a move in the right direction. But I'd rather see the farmer discovering a NEW plant type. Do the veterans here remember how happy you were to find quarry bushes in your cave? Imagine, if you will, a set of rare plants (that you cannot start with) that the farmer/herbalist could discover. The existing plant mod (here) could provide many inspirations for this.
Plants that could replace wood, flux, potash, pearlash, sand, etc might be viable finds (balance needed). Urist Hoemaster has discovered a Tower-Cap seed!
The suggestion of a legendary ambusher mood has sparked a bit of debate. I agree that the tangible benefit of a legendary ambusher is that you have a dwarf ninja. But in keeping with the item creation theme, the dwarf could, for instance vanish, return later, and proceed to head to/convert a leather workers shop and make a (insert semi or megabeast name here) cloak, boots, quiver, etc.
Such an item will likely have no more benefit than an artifact scepter.
Replacing some mood items with a large quantity of a normal consumable resource (I mentioned 25 plump helmets) would just be a conceptual placeholder.
So in conclusion, my model suggests:
1)Any skill can be boosted to legendary by a mood
2)All moods generate an item of some type
3)The primary benefit of a mood is the skill, not the item