Challenge: Have the Sorbs become large enough so that they get their own country, and have the Czechs just be a very small minority in Germany.
The first part is quite doable (one would wonder why it wasn't done IOTL after 1945, Stalin should have love the idea).
The second requires a weaker Czech cultural tradition. Eliminating Hussitism might help.
For a later POD, have the Bohemian phase of the Thirty Years Wars be longer and more-fought, and Bohemia in general be more of a battlefield during the whole conflict. That would both ravage the population severely (with repopulation done by people that would assimilate to German identity) and likely foster a more thorough Hapsburg policy of "Germanization", or, better said, of deculturation against Czechhood (see as tantamount to subversive Protestantism).