What's wrong with Khan Academy?
Don't get me wrong; Khan Academy is, in many ways, not bad. It's simple to use, accessible, and well-known among students. The amount of content is not the problem, it's the way in which the content is presented that I have a problem with.
Over the years, as I've taught students with weaker math skills, I've noticed a common theme. Many of them have trouble making algebraic concepts (including “pre-algebra” concepts like order of operations and fractions) make sense. What I eventually realized is that many of these students don't think algebra CAN make sense. They truly believe that some people “just know” what algebra steps come next in a particular problem, and that they don't.
For example, a student might struggle to solve an equation like this:
$$ 2x-5=9 $$
To students familiar with algebra, this is likely an extremely easy problem: add 5 to both sides, and then divide both sides by 2. But to a student who struggles, they might recognize that they need to do those two operations but not understand why they need to be done in that order.
It is possible to explain this using order of operations (“PEMDAS”), but many math instructors simply do not understand that it is something that may need to be explained for some students.
What Khan Academy does well is showing students what to do to solve a problem. A Khan Academy video for the equation above would definitely tell students to add 5 to both sides and then divide by 2. But it doesn't tell them why. Without the why, that same student will be just as lost when you ask them to solve
$$ 3x+7=4.$$
My vision for a new KA-like resource would focus on building that understanding. It would give students the tools they need to build on that understanding. And I have a bunch of ideas for how to do it.