Ask HN: What should I learn and write about next?

As a front-end developer what should I learn next to improve my skill set. (and I also want to write articles about it while I learn; like a journey)

I use (everyday): REACT, REACT-NATIVE, REDUX, WEBPACK, CSS-FRAMEWORKS(Boostrap, Bulma, Materialize etc)

I know (not using everyday): NODEJS/EXPRESS, BASIC DBs (MySQL, Mongo. etc), Firebase / Firestore, ROR

Any Suggestions?