For the last few months I've been interviewing for a variety of full-time jobs. Over the last two years, I've had a lot of time to think about what I want to work on, and the best answer I could come up with was "interface design, in every sense." In any design, the only parts that last are lines we draw between the different parts. The people and software on either side of an interface come and go, but the interface remains. The more foundational the interface, the longer it will stick around (see POSIX and TCP).