What are the different fields in software/web application development for freelancers?

For example c# or java is not a field.

data warehousing and machine learning are.

I'm on my way to become a junior c#/asp.net developer. I'm more interested in backend development and work as a freelancer full time n down the years run a small customized development shop.

What areas/fields/specializations/niches are there which I can focus on?

Would be greatful if someone can point out what fields are available for a c#/asp.net backend programmer who wants to freelance to focus on.

Language is not the issue. I need to select the field and can learn whatever language is needed for it for the required project.

What i know is c#/.net or asp.net or things like web application development are too vast and include varies fields.

I just want to know the individual ones to focus on.

Thanks everyone.