By 1890s, most obvious places are already taken or already in someone's sphere of influence.
In such a scenario, the United States would only want an open port where American ships can trade. Historically, the United States has the most connections with Canton since that is where most Chinese-Americans came from and so the most personal connections that could be exploited. I believe the US would want something in the Pearl River Delta.
Of course, there is already Hong Kong and some of other nearby places are already taken (Shamian Island). However, I think the US could grab one of the other areas on the south bank - perhaps Xiangshan (modern Zhongshan) or Zhuhai. If not that, perhaps somewhere on Hainan. Whatever is given would have to meet British (and possibly French) approval given their interests in the region, but this probably would not be too hard to get. By 1890s, the US was on pretty good terms with both - a diplomatic solution is presentable.
With access to the Yangtze region through the Shanghai International Settlement, this would give the US an area to develop in the other major economic area of China.
I imagine American administration of any concession would be similar to what was eventually given to the Philippines. There would be a US commission sent. A US governor-general would be established, but there would be a civil government and native Chinese would be allowed to hold office and rule themselves within the confines/limits set. The US would probably want to show the world how an "enlightened" nation would administer Chinese people, and with the idea that American ideas could be used as an example for the rest of China to modernize. Even if the US is more imperialistic than idealistic in this timeline, I don't see the US actually wanting a European-style colonial empire. It sounds like this is just a means to facilitate American trade absent the Open Door, but US interests and policies are still geared to wanting China to be sovereign.
