Let's start this custom showcase with a bang, we all want to see new classes in Hearthstone but understand that it would require a lot of work to make cards for those classes for all the different sets and have them be balanced. I was thinking of a few ways Blizzard could make this work and here are the 2 best options I see...
Option #1: Hero Class cards - A new take on hero cards, these would equip automatically at the start of each game replacing your chosen hero with the new hero that you have put into your deck and replace all class cards in your deck with ones from the class of the hero you equip at the start of the game. This allows for some deck building still with neutral cards and consideration going into how many cards you want from the new class, it could even replace the common, rare, epic and legendary class cards with the ones nearest to their mana cost from the new class of the same rarity, this would make for more strategic deck building with these cards as you can pick how many rares, epics and legendaries you want from that new class.
Option #2: A new Whizzbang type card - Pretty simple option here, have a new card like Whizzbang that gives you one of 3 decks for each new class, so for example you could have 3 pre-built Death Knight, Monk and Demon Hunter decks that you can be given at the start of each game to play with. This means no deck building around the new classes but does mean we still get to play with them while not needing to own legendary class cards ourselves and you would only need to get this 1 new legendary to play all the new classes instead of option #1 where you have to get the one you want by chance. This is also probably easier to balance than option #1 and Blizzard wouldn't new to create as many new cards.