Hi there! I think the website is pretty good, and I have a few improvement suggestions:

• After completing a practice, there's no retry/replay button. The user has to refresh their page if they want to do that practice again, which is a little inconvenient 
• You could have a question/problem of the day where the user can win points and refresh their old knowledge
• Have a jeopardy, or trivia game! Users can select which topics they would like/have studied, and they can compete solo, against their friends, or even other users on Khan Academy. Users can choose between recreational or competitive, which would include  weekly/monthly/all-time leaderboards and badges.
• Mini game adventures. The user must correctly answer some questions. Each time they do, they win a small piece of a badge. After all the rounds are complete, the user can enjoying piecing the badge together, and win it! You can have different difficulty modes, where the number of pieces varies. This could be a solo game, or against/with other Khan Academy users
• A variation of above: instead of a badge, you could have a puzzle. Each time the user is correct, a puzzle piece appears. They have to connect the pieces before a timer goes off. The puzzle could be some of the Khan Academy avatars, or anything else.