Khan Academy is already fun but it can become even more fun if it adds more gamification, the goal is not user retention, but user satisfaction. It should not be addicting or fear-inducing but it can be a little more fun, so I have a few low-priority features to recommend.

1. Home Screen Widget

• Duolingo offers a resizable home screen widget to remind its users to study. Adding it for the first time rewards the user. Khan Academy can reward the user for adding its widget with a few energy points instead of hearts.

       2. Studying Heat Map

• An optionally public profile page element: a monthly or yearly calendar representing the user's amount of studying, similar to Duolingo's calendar widget or GitHub's yearly contribution heat map.

       3. Achievements

• Achievements are designed to make our brain think we're doing something productive, even if we just failed a Course Challenge and leveled down 5 skills but earned a consolation prize (badge). I'm not sure if that is possible for an App in the Education category but I think it would be interesting if Khan Academy's badges could be unlocked through Google Play Games/Apple Game Center achievements.