Say the Habsburgs win the 30 Years' War early on? Not totally, but decisively enough to establish control over 'kaisernah' Germany and the formerly Protestant middle. Over the next decades, they establish an increasingly centralised, absolutist administration (and royally fuck up Bavaria once they decide to object). German identity is centred on the emperor and the counterreformationist church, and the northern fringe is increasingly seen like the Netherlands or Switzerland, once part of the Empire, but now a past memory. Prussia is first drawn into Swedish control, then taken over by Russia and turned into a German-speaking satellite among the many such states and statelets of the Baltic Rim, no more German than Swedish Pomerania, Danish Holstein or Dutch Frisia.