Assuming you have enough qualifications to start at the entry level:
Basketball/other sport coach
Traditional career to CEO
Writing, music, and entrepreneurship are scalable at every level - pay is tied to output and demand, there's no billable "per hour" component. Beginning careerists, academics, Wall Street lifers and coaches are paid a fixed salary, but as people move up the ladder their salaries become more tied to output and less tied to a per-hour consideration.
In which fields are the skills necessary to be successful relevant to the material? For example, to become CEO you must master office politics, and to be a successful coach you have to be able to recruit players. I guess these are part of the job description in each case.
My initial reaction is that difficulty depends on two factors - the size of the field and the amount of luck required. It's difficult to be a lucky coach or a lucky academic, but extremely easy to be lucky in finance, almost as easy in music, a little harder in writing. There is also a political aspect - consider which fields we would expect someone's son or daughter to have a leg up on the competition.
Entrepreneurship may be the easiest but the perceived risk of entry is the highest.
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