Very very fun, would pay money to play even as is.
One thing I would be sure to look into is visual indications of card/rune upgrades. Part of what makes the gameplay loop so satisfying is the process of collecting your deck, so making it visually "shinier" as it goes would make it that much more rewarding.
Also IDK what your plans are for this direction, and I assume you have some, but just going by the card art alone there's some potential for some really cool story/theme elements. There's no need to make it a story-driven game or anything, but it would be a really nice finishing touch to build on the curiosity your game already gives through its art etc.