Well, sadly writing isn't the most profitable thing ever unless you have a REALLY high reputation and a good network of people that constantly buy your services. Otherwise, it can be a tough market to earn in because everybody tries to be a writer... even if they can't write well. So the more competition there is, the lower the prices become.
I think that the biggest earners are technological skills. Anything that has to do with it. From SEO, website creating, coding, program developing, app developing, designing websites and so on. People are always trying to increase their traffic and that's why SEO is lately super popular... coding and the rest were always high earners.
I do translation and earn pretty nicely. It depends on what language you can translate from/into though. If a language is in high demand, you will get flooded with work offers.