Subdomain vs Subdirectory vs New Domain

I currently run a service based business and have decided to broaden my audience based on my knowledge and authority. I am going to begin adding in content marketing for other pros within my industry and believe this will do two things.

1) Help build authority and credibility to my current/prospective clients
2) Lead to the ability to introduce affiliate links and/or educational programs to sale.

My main question in working thought this is whether to use a subdomain, subdirectory, or a new domain all together?

Thanks in advance.