Favorite Cards Filter-function for Deck Builder
Add a Favorite Filter button ( 1 ) similar to Colors that filters your collection for cards marked as favorites. It should use all the avaible Filter settings but only show favorite cards that match these.
There should be a Drag&Drop Area (2 ) where you drag cards to add them to your favorites or when filtered for favorites, drag them there to remove from favorites.
all the needed tech is already there, should be simple to implement.
added benefit for WotC: you can collect the favorite lists from clients and get a favorite list of the playerbase. easy to see what cards are favorites of most players. you could use that for statistical analysis in order to make more cards and mechanics players like to play with or boost less popular ones.