The part I really enjoyed, and can be further explored, is the swapping of character types for making different types of attacks. Keeping this simple is a good thing in my view (for gathering players initially).
Not sure if the best path to more complex cards is, if having more cards or changing how the existing ones evolve with upgrades. Also the randmonness of attacks is playing a good simplistic mode here, which I like. Keep that I would say... if it gets too technical, it might spook some simple players.
It is true that if something becomes very complex it is scary, it is better to have something simple
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