This is something I like to do every so often as a hobby project, thinking of cool cards I would love to see in Hearhtstone.
I also once entered a Reddit Design Tournament and made it to the 3d round with the cards right below (in order of rounds from left to right). Each round had a theme to design around:
Next to that I also made a complete set of cards that would be in the theme of The Icecrown Expansion which I created in 2016, in 2017 Hearthstone added its own expansion The Frozen Throne into the game, containing several of the same characters I used in my custom expansion. Which made it possible to see how Blizzard translated the characters into the game, compared to how I did. Click here for the full set.