Yep I understand that the game will play a certain way with hand as health and that creates the tension in the game.
Some random thoughts I had about additional content:
Junk cards you can add to your opponents deck or they can add to yours.
I like the themes of the different opponents. I think with the exception of shields there's not a lot of cards that get better if you have more of them (I'm thinking of Shield Bash and the spike shield card). Perhaps these two things can be combined in some way. For example if you drafted several rogue themed cards they got stronger. Or just other cards that get better if you have more of them in your deck.
Leveling up cards was mentioned which I think is a good idea to explore. I also like the idea of leveling up your hero with some passive abilities as you advance. Similarly you could create a few different heroes that start with an ability of some kind that players could try to build a deck around.
After playing for a few hours it seemed like there were a handful of cards with interesting effects but due to the nature of the game (hand as health and small integers) the game was over too quickly for them to have a big impact. It felt like advancing in the game was reduced to playing cards with immediate effects on the board and so those more interesting cards were almost locked away because you died the turn after you played them.