Not sure how many people like this idea, but as a reward for completing a world, a wizard should have a chance to battle/train with their professor. If they win, they should get a powerful item they can use (i'm thinking gear). It can be something already in use, but that would be an extra incentive for them to keep playing and look forward to their next training session.
Also, at a certain point, make it possible for wizards to challenge other school professors...not just in Wizard City, but in other worlds as well (like Wysteria). This can be for a fee like 250 crowns, and not allowing henchmen or treasure cards to be used...just skill and what your wizard has learned.
You can also make the professors dueling ability adjust to the wizard's level. One last idea...make it possible to duel each professor for one extra training point. Make the cost like 1000-2000 crowns, but once defeated, you get that training point forever to learn more spells.
Also, a new badge could be made if you defeat all the professors...call it head of the class or teacher's pet or smarty pants...something cool like that...I see this as a really good way to keep the game interesting and growning for all wizards.
