Personal feedback on the Mastery Challenge
The concept itself is very good—a quest system linked to the event with 1 to 4 optional and selectable restrictions and a reward based on the level achieved and the number of restrictions chosen—it's interesting and offers a challenge to both novice and experienced players, who can adjust the difficulty.
The big disappointment is the reward itself, or rather what it allows you to buy: 'Card Sleeves' a kind of skins just to make them look cooler !? Honestly, I don't see why anyone would waste their time trying to complete this type of challenge for such a ridiculous reward.
If you set up a real marketplace with mastery gems that can be used to buy useful items (like corrupted gems), such as pigments, golden epic equipment, familiars, feats, ... or rare & valuable potion like gem hunter, golen epic potion...or even convert mastery gems into Tiamat scales, then yes, it would become really interesting and attractive....