Focus on tech. with long life-cycles (as in spend 80% of your learning time on it) and the rest to get familiar with the latest cool things. As an example in a world where most teams don't have a DBA being a go to Person for say Postgres is a valuable position to be in. The 20% time also needs to be strategically allocated spend it on something that is likely to have staying power. Also don't spend it all in one area as in learning React and Ember and Angular and Vue or spending it all learning all the KV stores.