Ask HN: How can I identify which problems I should solve in my spare time?

More specifically, web projects.

I'm a front-end web developer who's looking to brush up on back-end development. Stuff like Rails, Node.js, Ember.js and so on.

I've been trying to work out what to build next, but I have writers block and it's very frustrating. I'm not expecting anybody to give me an idea, but I was wondering if there's anything I could or should be doing to muster up some ideas.