This has a pretty good gameplay loop (although if you get bad RNG a lot, it can be quite annoying. My enemies top deck 20s and 21s WAY too often).
I think you should be able to re-fight lower difficulty fights within the same run (the game has so much RNG that the odds you'd get through all of the fights are INSANELY low). I think that adding more characters WOULD help mitigate this (since you'd have more low-level encounters early on).
Some other skills like "forcing your opponent to draw a card" or "reshuffle your hand" could be some good powers.