Request: Action that allows adding and removing cards from the collection
This will allow developers to make everything from customized card rewards based on variables to custom shop-like modes where players can buy cards with far more control than the Market provides, it will also allow for players to exchange cards for materials to use for different purposes, again generally with more control than Market or Sets currently provide.
Really the inclusion of Remove Card and Add Card to collection as an action for effect builder, will be able to function as a pseudo-replacement for having multiple currencies, the ability to dust cards, and having a trading system if you so desire.
Over all the ability to remove cards from user's collection is a feature that is sorely missed in any capacity.