Shanghai and Beijing are loaded with 5* hotels from international chains that are excellent, but like the OP said, they are probably not too different from 5* international chain hotels in most other places.

One suggestion for Shanghai would be the Ruijin Hotel in the French Concession, which is much more characterful than most of the modern steel and glass towers that abound in Shanghai. All of my experiences in Beijing are the type of 5* hotels you claim to wish to avoid. One thing I will say about Beijing is that traffic is terrible, so location is an issue. Among the deluxe international places in Beijing, I'd say that the Hyatt and the St. Regis have the best locations for proximity to most of the stuff tourists would care about within the city.

In Xi'an, I was pleasantly surprised by the quality of the Sheraton, and if you have SPG points, it's great value as an SPG cat.2.