bonus experience for completing quests/achievement
leveling up could be expedited by awarding experience for completing quests. Doesn't have to be a whole lot but would allow players to level up quicker, which many will find helpful once more sets are released that become available in career based on your level.
Something similar could be done for achievements, as some require a huge amount of effort just for a single copy of a card. Maybe dividing them up into tiers and granting experience based on what tier (Tier 1 = 200 or 300 xp, Tier 2 = 500 xp, Tier 3 = 1000 xp) and so on, with the tiers being determined by just how much time has to be put into the achievements