Okay, right now level 50 is the top, I get it. What's left after questing is done? Right now, there are two choices: help others to level up, or go out phishing.
Why not have xp continue to accrue? Instead of leveling up, you get training points. Wizards can learn new spells. Certainly it will be a challenge to learn all of them. Of course, the decks would not change, so there is no advantage there. It'll just take more finesse to use the deck.
To take it further, and maybe too complicated, it could be that you earn different types of xp for different types of bosses/creatures. E.g. for every balance boss, x amount of points; which can then be traded for balance spells.
It's a challenge outside phishing for equipment and bragging rights. knowledge instead of stuff. Count me in.