I concur that unique mods, and for that matter unique consumables in general, are a good idea. Other possibilities for artifact items:
A portal supercharge, usable like a medkit.
Ultraportable nuke. Can be dropped down a staircase to clear out the level before you enter it, with obvious applications.
Nuclear mod. Turns a plasma rifle or BFG into the nuclear equivalent.
Armor total upgrade mod. Adds all three mods to an armor, regardless of Whizkid.
Weapon total upgrade mod. Adds four random mods to a regular weapon, or twelve random mods to an advanced weapon, regardless of Whizkid.
Rapid fabricator. Generates huge stacks of every kind of ammunition, enough of any given type to fill even a backpack, around the player.
All of these would be destroyed on use, of course.