Ask HN: Picking fresh technologies for a slightly complacent programmer?

I have 8 years of professional experience, and for the last three I've been doing Python exclusively for backend development. Distributed systems mostly, very little interactive stuff, mature product, large company, complex features taking multiple months to implement, working solely using an internal framework... Before that I was working in DevOps and Java/Scala projects in similar domains, but on smaller projects.

At first I got really bored with Python and our stack, but now I realized I became complacent. I like my job because it's challenging, and I like the things I do (not to mention it pays really well), but I want to hack things together again on my spare time which give me a shorter reward loop, and eventually touch on some DevOps on the long run. I'm not completely stuck, don't get me wrong, I read blogs, follow conferences etc. and have a few modest ideas for side projects, but I would like advice on something the cool kids are using, in similar areas I'm in. What would the fine people at HN suggest, Java with the latest Spring stack and/or Akka? Elixir? Go?