[nojvek]

Compilers - every programming language uses this.

Databases - Btrees, indexes, distributed key value stores. The world runs on databases.

Graphics - if you like games, how a 3D scene is rendered, photo realism with Ray tracers, GPU pipelines and OpenGL

AI and Machine Learning - tons of fascinating problems and algorithms.

I’d say focus on the basics. Hash tables have remained mostly the same since they were invented. C still uses pointers, the basics are fundamentally so powerful that they are kind of eternal.

When I hire someone, i’m looking for someone with strong fundamentals. They understand the basic datastructures, algorithms and how a computer works.