Relevant to my interests! Here's some stuff I would have enjoyed learning prior to learning it the hard way:
a) Databases. What's the right way to work with them in Golang?
b) Suppose you're building an API which has some shared state. (Like, I don't know, a stock exchange's order book or something, to pick an example randomly.) What's the best way to model that such that you can access it concurrently in an arbitrary number of simultaneous HTTP requests?
c) Supposing that one understands goroutines at a Golang 101 level, where does one go from there? I ended up having lots of fun on e.g. fanning out data incoming from N sources to one channel back out to M consumers which each need a copy of everything on that channel. This felt excessively painful to me. How do I do it in a way which is not excessively painful?