The threads in here about honestly confronting the finite time available and defining goals and understanding why you want expertise are both reasonable and important to think through.
But in terms of actually picking what to focus on, I'd suggest looking for under-explored intersections of domains that are interesting to you. This is for two reasons:
- If the particular intersection is not field with a large number of people doing real work in it, the "bar" for being an "expert" is lower. In a well-developed area, even after doing quite a lot of learning, one might not be an "expert".
- The intersection can give you a view on those adjacent fields which may reveal other interesting opportunities, and may make you at least fluent or productive in those areas.
But in terms of actually picking what to focus on, I'd suggest looking for under-explored intersections of domains that are interesting to you. This is for two reasons:
- If the particular intersection is not field with a large number of people doing real work in it, the "bar" for being an "expert" is lower. In a well-developed area, even after doing quite a lot of learning, one might not be an "expert".
- The intersection can give you a view on those adjacent fields which may reveal other interesting opportunities, and may make you at least fluent or productive in those areas.