I think there are a few ways to improve Wizard101.
One thing I have always wanted to see was a night/day feature in Wizard City, Krokotopia, and several other worlds that fit well with the feature.
Updating the commons would be something else I think would look really great as it makes the area more visually appealing considering that's where a lot of people spend there time whenever they talk to other wizards.
Add a 7th character slot so people can have a wizard of every school.
In the character selection screen, once you click on a wizard, the background turns into a theme for the school that the wizard is in. I know people spend less than 30 seconds here, but it would still be a nice little add-on into the game.
A New Side World: We have Wysteria at level 25 and Grizzleheim at level 20/40. Adding a new side world for level 100+ would be really fun to do while we wait for the upcoming part 2 of Empyrea. An idea I specifically have for a new side world is related to Pirate101. I think Skull Island, Cool Ranch, and/or Valencia should eventually be included into Wizard101, like Marleybone and Mooshu were added into Pirate101.
