If you don't have a favorite field or a skill, you'd better choose a hot field, but don't touch something that requires qualifications, such as health, securities and financial society, which all require qualifications. Don't play with these, because your brother can't publish content at all, and even if he can't, you haven't recommended it. Instead, you should analyze what is a hot field. If it is hot, you'd better give the dog a bath.
Therefore, I recommend you to choose some hot areas. The domain of each platform is different. It depends on some data analysis of the platform. You can tell me which field you want to do, and I can help you analyze it, or which platform?