Ask HN: What should I learn next?

I've been developing on the web for about 8 years now and have cultivated a set of skills that make me very employable as a contracted/agency web developer. I'd like to break out of that a bit and am considering "The Next Big Thing" (TM) that I should learn. Last year, I took an 8 week, full-time class in JavaScript and it was a lot of fun with the added benefit of being very useful.

Just based on who I know and meet, I would guess that my current skill set is quite common so I hope this is useful to others in the same position. I'm not worried about my future or anything, just want to branch out.

Current skills are:

- LAMP development (solid in PHP, MySQL, WordPress, picking up Laravel)

- JavaScript (front-end interaction mostly, basic Backbone, I'd call myself intermediate here)

- Basic CLI (I use it regularly, not scared of it [anymore], unlikely to make make big mistakes, but not a power user by any stretch)

- CSS/Sass (power user)

- HTML (sounds weird to say but I use it correctly, consider the structure, understand how it related to the DOM)

- Node (minimal, learned it during the class and really enjoy it but don't find a lot of use for it day-to-day)

Here's what I'm considering. I really want to take my time with a single one, spend a year really developing into something I use regularly:

- Command line scripting

- Dive deeper into JS, including Node

- Dive deeper into modern PHP, moving away from CMSes

- A completely different "back-end" language like Python, Ruby/RoR, Go

- A completely different environment like iOS, OSX, Android

Thanks in advance for any guidance you can provide!