As you earn new Expert cards, you'll start to have a smattering of Expert cards across all the different classes of Heroes. A good way to make the most out of playing for free is to pick one or two classes to focus on, then craft some new cards for those classes. Do this by "disenchanting" the Expert cards from other classes, then using the Arcane Dust to craft new cards for the classes you want to focus on. It will take disenchanting higher level cards, or a lot of Common cards to make even one new Common card, but the advantage is you'll be able to choose exactly which cards you want to create.