It's hard for me to to frame nationalism in America because America is not a nation and any kind of nationalism is intrinsically subversive - very much like in Russia, so to speak. Usually the alt right speaks about white nationalism, but that's implicitly a separatist project, so it's anti-american or at least un-american. If I was to fix America as it is I would go for an Imperial kind of state, where the task of the government is to try to manage the contradictory requests of the various groups. Such a government can't be democratic however, because in a democratic framework you only get the various national and identitarian groups competing for the limited resources of the state. Christian nationalism however is the most solid proposal for an American nationalism, because Christianity is truly the only common ground for the vast majority of the country. It should not be a white nationalist movement however, it should aim to create an American race, which implies race mixing to a certain extent. Not something the alt right would gladly accept. But an ethnos - a nation - must be from a common ancestry.