Ask HN: I'm a generalist developer I'd like to specialise. What should I choose?

When I think of specialist developers, I imagine being specialist in a business domain. This is not to say that being an AWS expert does not have its benefits. But if you do want to specialize, I will suggest going with a business domain. For example, if you are ReactJS specialist today, what guarantee do you have that ReactJS will be hot tomorrow ? Who knows. But if you are an expert in say cryptography, security, finance, healthcare etc, I will bet they are not going anywhere anytime soon.

My 2 cents. All the points I made are certainly arguable.

Languages:

At some point in your career, the languages that you know stop being as important. Once you know 5-10, you'll pick up whatever your current company uses. You may have favorites, but beyond a certain point further specialization has marginal benefits.

Cloud architecture:

A good skill to have. Make sure you understand the nuances of distributed computing. There isn't a huge difference between the different cloud providers, though. However, there are enough subtleties with each that some specialization pays off.

Music streaming:

Not a bad business domain for specialization, with very interesting algorithmic knowledge required. There is a lot of hard stuff here. iOS is probably too specific here.

Other stuff:

The biggest demand right now (that I've seen, so YMMV) is for machine learning specialists. They are going for about $2M per in acqui-hires, about 4x standard dev. That is a very hard thing to specialize in without a math/research background, but learning the ins/outs of deep learning architectures can pay off fairly well even without a PhD. Jumping from web dev to that might be tough, but I've also seen some demand for web based ML visualization and interactivity. Might be interesting for you to combine with music.

Pick your favorite (the area you tend to find yourself spending more time on) or the one most in demand, high paying, or secure (whichever matters to you).

It seems Machine Learning / AI / Data Science (though maybe it's "maturing" after becoming such a hot field), mobile development, React + Redux, etc are what's in demand these days.