[tedyoung]

You haven't said what you currently know, but here's some ideas:

Security - even just understanding the various protocols and limitations of things like JWT would be useful.

Automated Testing - I'm a Test-Driven Development advocate, but I also know that's rarely easy to incorporate into existing codebases. However, the ability to write good, small, focused, developer tests is a very useful skill. And not just because of the tests, but often because it helps you implement functionality in smaller steps.

HTTP API Design - Especially since it involves designing protocols without you realizing you're designing a protocol.