I think one of the classes I most miss was Graph Theory, from Prof Estela Maris, at my undergrad level. It was extremely interesting and very theoretical, but quite useful in real life for some problems.
The proves of some properties of graphs were the part that demanded most of me, applying specific math knowledge for proving some properties of some graphs, i.e. trees, digraphs and so on.
I think most of the online courses I’ve seen so far they forget to tackle those kinds of data structure, and as another professor of mine used to say, if you forget the data structure you are halfway down the hill already.
So my recommendation is to study Graphs Theory and math proven, which certainly will be useful at some point.