This has been covered before, but it deserves to be repeated and upvoted as much as possible so that the powers that be pay attention:
Khan Academy is AMAZING at teaching math. Everyone who uses Khan recognises this. But the awesomeness of Khan takes a nose-dive after the high-school topics. Why? Because there are no exercises for the college level math topics. PLEASE focus on what is so special about Khan: the math, and prioritize adding math content - especially exercises for advanced math content.
I 100% support the mission of Khan to be a free educational platform for ALL subjects. I donate to Khan every year in support of that mission. But what makes Khan special is the math, and I beg you not to expand at the expense of the very thing that makes this website so special.
In conclusion, please add more advanced math exercises! Thank you :)