And though not sure if you're interested, my current ideas to improve the balance (also the whole gameplay) are:
- Offering 2 card rewards each round, this will help lengthen the deck-building process, which is the core of a rougelike game, and it will also improve the chance for players to get better cards (suggested by TheMixedMaster in Ludum Dare comment)
- Increasing mana recover from 3 to 4, this will allow players to play more cards each round, adding the chance for them to make decisions, this will also reduce the difficulty
- Increasing all units' health, this will give players more tolerance on bad luck, and will make each round longer to give players more chance to play permanent attributes modifying card