Assuming a "100y-ltr utopia" where there's accessible online textbooks and resources for everything, Which course/school/subject would still be non-optional and worth taking?
[Given an extremely skilled/determined autodidact.]
[For obtaining knowledge instead of license.]
Antiexamples: programming, maths (eg Ramanujan), law, philosophy.
Think woodworking, martial arts? Many more I'm sure.
I don't think this will be a very big list, though. You should be able to learn almost anything these days. I don't feel inhibited by anything except the sheer depth of some topics, and lack of my own time. I pretty much feel like I can learn anything.
Almost any topic can benefit from a good instructor, though, especially if it's one-on-one. They can steer you better than you can steer yourself as a newbie.
Is there a purpose behind this question beyond curiosity?