Which programming language are you are most interested in learning, but you never got around to doing so?
For me, Haskell was that language for a long time. Mainly to understand a pure functional programming paradigm.
I managed to spend quite a bit of time learning Haskell over the last two years (the irony - I never actually wrote Haskell code, I'd take all the examples and re-implement them in python, so perhaps its wrong to say I learned Haskell, more like I understood functional programming better - which was my main goal anyway).
Today, if I wasn't learning Python, I'd love to take a deeper look at Rust. Whatever little I've seen looks really great, and to have a safe language with performance close to C sounds fabulous. If ever I need to do high performance work, I would jump into Rust in a heartbeat.