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.