Ask HN: What CS or Software Engineering subfields are worth specializing in?

I’ve been working as a software engineer for about three years, primarily developing a GUI library for embedded systems. Recently, I’ve been considering pursuing a master’s degree or finding another way to specialize in a field that is likely to remain in demand and not be replaced or lose many jobs due to AI. I’m interested in identifying areas of study—aside from AI itself—that are valuable now and will continue to be relevant in the future. What fields or specialties would be worth investing in?