Since these new slots will only appear on gear post-U17, I suggest marking items that don't get a slot (the other 95%) as unslotted, and adding a recipe to the epic alter:
pre-U17 item + something (plat, dragonshards, etc) = 5% chance for appropriate colored slot / 95% unslotted
Recipe can only be tried once on pre-U17 items, never on post-U17 items.
With a 10k plat cost, you could probably hear the whoosh of all the plat disappearing. (Obligatory conspiracy theory: this recipe will be available but only with a store-bought item, and can be tried as many times as you can pay for)
And I completely admit this is a selfish suggestion, but I'm drooling over the possibility of adding a red slot to my holy burst icy burst of mangling/mauling weapons.