Awesome game, I've been coming back to it for weeks now. For a challenge, I've tried different game modes. You could add these as achievements for beating with these restructions:
1. Only pick hearts (more strategy when positioning as half of your hand is pawns)
2. Only pick pieces (pretty easy until the last few levels)
3. Only pick pieces and finish a run without losing a heart (pretty hard)
4. Only pick hearts and finish a run without losing a heart (Haven't beat it like this yet, takes a lot of RNG)
A lot of people get confused about which spaces the enemies attack. I don't think it can be a simple hover-over tooltip as the hit markers also change when some enemies die. I think a solution to this would be to have a toolbar where you can read about each enemy, or just put an infographic in the description of your game to refer to.
The game seems perfect as it is, I would enjoy more levels (some having obstacles that can't be removed for example), possibly different map shapes, different enemies, different game pieces (a random card that changes every turn) and different bosses. More ideas include a timed mode (either a certain amount of time per turn or per level). I have more suggestions that should be easy to implement.
You could even have power-ups, such as a one-turn shield, or have your piece attack in a square around the player. These powerups can be offered after beating a level as a third option, rather than choosing a piece or a heart. They would act as playable cards from your hand.
Also personally, I really enjoy the look of the game pieces more in this demo than in the updates you've been posting on Twitter (no offence). The demo's graphics are sleeker, more compact, and honestly look more finished.
Just my opinion. I don't think you need to spend a lot of time on graphics, but rather in more variety in the game.