I agree that the reward card system right now is just way too generous and cards are being printed and given out at a furious pace. It feels like yesterday when the new reward cards that include the furious chicken were released and they are already 50% out by now with the limited player base.
There are plenty of things that can be done to fix this. The way I look at it, reward cards should mostly help starting players to get a bigger collection without them having to spend all that much. Since the game gives a natural drive to 'pay-to-win' / 'pay-to-earn' getting that initial hook for new players is probably the most important.
Make the season longer and more difficult to actually climb up to higher levels. I wouldn't mind seeing a 1 month 1 season cycle which instantly would halve the pace the reward cards are given out from season rewards.
Make new reward cards require a lot more cards to level up to higher levels. (even more than the untamed cards) Low levels should not require all that many cards as it benefits new players, getting them at higher levels should always require those that want them to buy them on the market. Right now I already managed to grind out Level 6 level on many of the newest neutral cards that just came out.
Give out less reward cards for each league. If right now 1 card less would be given in Gold, 2 less in Diamond. Nobody would actually bother so much while it would make a difference in the rate they go out.
Another idea I had is to somehow slotify the daily quest rewards instead of getting them from opening quests. I'm sure that would make it possible to give out less while at the same time make it a more enjoyable experience. A function to play those slots with DEC or STEEM can also be implemented.
Anyway, nice to see the team has a long term vision and doesn't shy away from making short term unpopular decisions. The key will be to balance everything and get te rewards down one bit at a time to make them more sustainable.
Keep up the great stuff !