The best subject is one that you know.

If you feel there are more income opportunities targeting the U.S, I'd suggest doing a site for Chinese-speaking Americans.

I suggest sticking to your native language, because then you can create a site that is professional and will be taken seriously. Language will not be one more issue to deal with.

What subject do you know a lot about that would be of interest to Chinese-speakers living in the U.S.?

Think niche. I am sure there are sites already for news, life in China today, etc. You need to home-in on something very specific.

I think only yourself know what is best for you . You can try to seek for valuable information related to what you are interested in . If you want to make a fortune via your website, it takes proper plans to design whole. It will do much good if you pay attention to your web content and online marketing, for sure ,anything useful that can trigger clients interest is worthy of efforts. Good luck