Poland seems easy enough, their entire country wiped off the map.
Someone from there gets here, wealthy starts a colonization society, helping every Polish person who will move, move.
It's expanded outwardly into other repressed Slavic peoples, the Serbs, the Czechs, Slovakians, (which in otl are a lot of Texas), The, Turks, Russians, and Austrians are happy to oblige.
The German states are unified under Austria, the Hungarians, Czechs rebel.
The Hungarians get a similar deal to otl, the Czechs are further encouraged to leave.
Eventually these groups descendants become the largest groups in the United States.
However as in our timeline these people assimilate into larger American culture. You just have more Slavic surnames in the Midwest and not very many German ones, but there's some.
Despite speaking English most of American ancestry can be traced back to Poland.