The new Mercenaries game mode launches tomorrow (though unfortunately with some bugs on launch) and in celebration, the HSMercenaries subreddit is hosting a contest to create your own Mercenary of either Chromie or Tracer. Submit by 10:00AM PDT tomorrow for a chance to win a $20 BattleNet Gift Card.
UPDATE: The submission window has expired! The winner has been announced!
You must design the entire kit of tools for your Mercenary of choice. Level 30 stats, role, abilities, and equipment are all a requirement. More specifics can be found in the Reddit thread and it's definitely recommended that you read it if you're interested in entering.
Creating Mercenaries on HearthCards is a little bit strange and it's also fairly hidden and far from perfect, but it's there. If you open HearthCards, you may notice the word "TEST" in red appearing above the list of card types.
This is actually something you can click to design Mercenaries. Click on the word TEST, and you will be taken here.
From here, you are able to design your own cards for Mercenaries with all the bells and whistles you'd expect. It has a few glaring issues. Namely that the artwork has a tendency to disappear when you try to add it to either an Ability or an Equipment card (it's still there and still interactable, but not visible), and the art will clip through the top of Ability cards. Card types will also not show up on Ability cards. While it's far from perfect, it's very much accessible and available if you wish to create your own Mercenary cards.
You have less than 24 hours to enter, so get designing!