I love this game! It would be awesome if there was a challenge to see how far you can get. I've found it fairly straightforward to beat the existing content. The game would have a lot more replayability if I could compete with myself to see how strong of a deck I can build.
A quick way to accomplish this would be to encounter a combat version of the practice dummy after the final boss. See how much damage you can deal in 3 hands or something to that effect and that's your "score" for the run.
It would be even more satisfying to progress through randomly generated encounters after the final boss with a counter to see how many battles you can win. Like Hildebrant's Cellar mode if you're familiar with the game For The King. Randomly generate the enemies and exponentially increase their health and attack power with each battle. You can continue the progression of battle prep battle prep like the existing content or it could just be battle battle battle until you die. Add a little counter to show how many battles you've won and that's your score for the run. If the enemies are growing in strength exponentially then I think the player would have to be vanquished at some point. I would play something like that many times trying to beat my high score. I would even call it a full game at that point!
A gameplay suggestion. I really like the staging/prep deck concept used to shape the combat deck. The combat deck feels strategy heavy and I love it. I would be very impressed if you could bring the prep deck up to the same level, so it feels like I'm trying to build 2 decks.
Maybe the combat deck can also influence the prep deck? For example: slash for 2 damage and draw an extra card in your next prep deck hand. There's already some churning possibilities for the prep deck. How do I trim the prep deck? Make it stronger? Combine your cards to dig for treasure twice? Draw extra prep cards at the expense of fewer combat cards? You've done an awesome job with this game so I will leave it up to your imagination.