Ask HN: What skill should I learn next?

I have 15 years of experience working as a developer. Last 6 years I have been focused on iOS platform. The last new skill i acquired is Swift. I am looking to add next one that make me relevant (or even advance my career) for next 5 years.

What should I learn?

I have a list of candidates of my own but if you can support one of the following skills or even suggest new one would be great:

(not in any particular order)

1. Python

2. Deep Learning (interested in its applications but afraid of Math)

3. Blockchain? (not even sure what I am talking about here)

4. Backend development