In addition or as an alternative to Unique Variants, I would like to suggest the inclusion of an "Enchantment" system. The purpose of this system is to provide a less-frustrating and more flexible means for players to obtain items imbued with the bonus abilities associated with Unique Variants.
Enchantment would allow players to purchase recipes allowing the crafting of runes, items that, when combined with a valid target item (weapon/armor/shield) would temporarily imbue that item with specific Unique Variant properties. Runes would thus be relatively common, tradeable, consumable items that provide a limited time benefit in constrast to the rare, permanent Unique Variants.
As consumable items, runes would require replacement or renewal to maintain bonus abilities, this would allow high quality, expensive runes to serve as a crown and crystal energy sink, one that is considerably less capricious, and thus less frustrating, than the UV system.
As tradeable consumables, runes could benefit self-styled "crafters" and "recipe-collectors" by providing commodities less prone to devaluation due to massive surplus of unwanted permanent items produced through UV gambling. For similiar reasons, they could help stabilize and maintain the value of common materials.
Enchantment runes could also provide increased flexiblity by: indiscriminate application to any items of a given rank, whereas UVs are much more difficult to obtain for certain alchemy trees, allow overwriting of one enchantment for another (greater flexibility with bonuses on one's preferred weapons at expense of re-enchantment).
We basically have this functionality in the form of trinkets, although current bonuses only correspond to armor UVs (status and dmg resists instead of +dmg and CTR). Perhaps, instead of weapon add-ons (and the name runes, which really doesn't fit with the space-traveler mechanical world of SK), we can have trinkets that impart weapon UV functionality instead of just status resists and extra health.
I like your idea for having such trinkets be craftable and temporary. Perhaps once you bind them, they can be used for up to 7 days, and unbinding them or switching to another trinket in that slot makes them dust completely. It would be fun to play with that kind of rotation of special abilities.