I want to do a book called around the world in Cleveland, OH. There are lots of ethnic neighborhoods--Greek and Russian Orthodox and Polish neighborhoods where the churches reflect the ethnic origins of the first residents. Restaurants with ethnic foods still exist.
I haven't been to every large city in the US, but I'd guess most of them (particularly in the old industrial northeast with eastern European immigrants and the western US with Asian immigrants and places in the US southwest with Latino immigrants) have neighborhoods which reflect the origins of these immigrants. I don't mention British and western European immigrants because I think these became what we think of as American and thus don't qualify, although you can visit places with lots of Tudor and fake British architecture.