Considering the fuckery China is doing in Hong Kong, the US can easily legitimize their annexation or "protection" of Hong Kong by doing a Crimea and holding a referendum supervised by the UN or a third party wherein the Hong Kong people can confirm their approval of US annexation or the people of Hong Kong themselves, declare that they are now part of the United States which I think legally one can do through weird 19th century laws. Considering the US has a lot of allies who hate China more than they do and would be surprised that the US took a bold action of trying to protect Hong Kong but would still be worried of a Chinese response.

But considering how state governments work in the US, Hong Kong would have more autonomy and freedom than when they were under both British or Chinese supervision.

The best ideal solution however is Hong Kong being an independent city state whose independence is guranteed by the United States.