Right now, as I understand it, there are not clear plans on artifact effects except in their nature for personal disputes, and because they are valuable.
I believe it is quite reasonable to apply some basic effects procedurally, and I think I have come up with a basic system for such a thing.
So, dwarf fortress as a series of spheres.
An artifact can be separated into a couple spheres, based upon its materials and what it is. If this means it has more than 3 spheres, 3 will be selected randomly. If this means it only has two spheres, a third will be added randomly. That will probably be the only way to add more unusual ones like Twilight.
A sword might for example be War/Metals/Something random.
A basic effect format that can be done, would be Effect/Cost/Target. Cost applies to the owner of the artifact (perhaps the owner of the room for furniture style items), and both Target and Cost can change effect scale.
For example:
Metal might be:
Effect: Repair damage to items
Cost: The artifact degrades slightly. 10x effect.
Target/Trigger: When something is hit with the artifact.
War might be:
Effect: Cause harm to a target.
Cost: The wielder feels pain 1/2 effect.
Target/Trigger: Everyone near someone who is struck by this object.
As a random third, because its a basic metal sword (maybe with spikes of metal).
Twilight might be:
Effect: Causes drowsiness.
Cost: Causes drowsiness in user (1/2 effect).
Target/Trigger: Everyone near the object when the sun sets each day.
So this metal sword might come in one of several configurations.
It might be Metal/War/Twilight
Repairs items, at the cost of making its owner feel pain, every day at twilight. This repairs them at 1/2 base effectiveness, whatever that may be.
It might be War/Twilight/Metal
A pretty classic magic sword. Causes harm to people it hits, while making its user drowsy. Half effectiveness of the harm.
Overall the basic premise of the system can generate a pretty huge number of effects, while respecting general themes.
For divine artifacts, a god could simply choose 3 of its spheres when blessing an object.
Its relatively easy to come up with a full list of these things for all spheres, but implementing them is the real difficulty.