Rather than skipping on to a new framework, why not go a bit deeper with React? Learn more about writing tests with React, for example. Dig into the deployment side. Review your methodologies - are you writing small, pure functions? Is your display code done in a nice, modular way?
Alternatively - how far have you dived into ES6? It goes very well with React+Babel; and it's a relatively long-term skill.
Or learn something totally different. eg. If you've never done python or bash scripting, learn the basics of those. Get a feel for different ecosystems, communities, idioms and language syntax. Doesn't mean you have to put them in production, but it will broaden your programming experience.
Maybe think of it this way: would you learn anything seriously conceptually new by learning another client-side mvc framework in javascript? Or are you just learning more variations of the same thing you already know?