Ask HN: What can I learn next that will have practical value as a senior dev?

I know all programming paradigms, imperative, procedural, oop, functional, meta, logic, array, actor, csp, dynamic and static.

I've designed and written large scale distributed backend systems.

I've lead teams in the delivery of big projects.

Now I feel I want to learn and grow even more. But I can't seem to figure out what topic or thing would further add practical benefit to my discipline.

What can I learn next that could allow me to innovate and simplify further the software I write, and have more impact?

I want something with applicability in backend distributed systems engineering.

I thought either machine learning or something business related? But I'm not sure.

Also, I'd appreciate suggestion for books or MOOCs to accompany the suggested topics if possible.

Thank You