About Mirrorworlds we are not planning adding card stones for it's currency.
You could add these items to the monster card dust / soul stone shop
It is related to monster cards by design, and it would limit the possibility to get these items, by the amount of soul stones in possession. Also, it could help new players to get ingame currencies, as they can farm monster cards from day 1 and sell soul stones. Actually the prices are quite low, but this could boost the demand a little bit.
And there is a huge imbalance between availability of same colour cars.
Some cards are farmable, just takes a lot of time, but others are almost impossible to get at all, and green cards require 65 cards to enchant to get to 5-star doing it the regular way.
Some blue cards are only available through missions, and unless you have 800-100 alt chars to repeat the quests, or constantly abaondon/start new quest for these mobs/cards it is really not reasobable to get/farm these cards.
e.g. Kasumille and rage goblin in aslan /red eyed bear in logar
In other regions you replaced these cards with "red level" cards, but these are still blue cards...