I am a TA for an undergraduate controls class. I would like to assign homework where students could get instant feedback, have access to hints, and be linked to videos where the concepts are introduced for each problem.

Khan Academy has these features. However, if the content does not already exist, it is impossible to create the content yourself using the Khan Academy platform.

Should Khan Academy implement this feature, I believe all parties (teachers, students, and Khan Academy) would benefit. 

Teachers would be able to track the class and collect metrics for how students are doing.

Students would be engaging in a pleasant learning environment.

Khan Academy could review the private course content and use it as a starting point for expanding its course library. Khan Academy has amazing content, but it is really lacking for undergraduate courses. I believe this could be the catalyst that Khan Academy needs to be more useful to undergraduate students.