Please allow us to mark our favorites cards to facilitate filtering/deck-building. I often reuse certain cards across decks, and sometimes I just want to review my favorite cards, not all cards that exist. My current solution has been to have an invalid deck with my favorite cards, but this doesn't make it easy to build new decks (my current solution doesn't leverage the other great combination of filtering tools that deck-building already has).

Even better, allow 3 sets of favorites so that I can have an "I always use these / high utility" separate from "these are extremely cool effects", etc -- or however a player wants to split them.