I really like these suggestions, thanks!
Challenges especially sound really interesting. They could encourage players to experiment with different card types/builds instead of always chasing the strongest setup, and the permanent rewards would give players something else to work toward.
I’ve noted the balancing/resource/building ideas too. I definitely want to add more long-term progression beyond just watching income go up.
For prestige, I’ve actually been thinking about keeping the collection itself, while resetting card copies/levels. Another possibility is letting players choose a limited number of cards to keep when prestiging, with ways to increase that amount later. I think that could make prestige feel rewarding without removing the sense of starting a new run.