I'd focus on learning data structures, algorithms and different architectures, when to use what and the trade offs. I say this because honestly, a smart person will pickup just about any language you put in front of them, but if they don't understand the choices behind architectures, data structures and algorithms then their value is far less.
Also, while IMO you should do the above I also agree with MalcolmDiggs suggestions too, you should use this time to experiment with different technologies and different parts of the stack to see what interests you. You may find you hate being a front end web guy, or that you love it. I'd pick an open source project and contribute/learn from it or write some self serving applications that do little things you or your friends would like.