What I tried to do here is to imagine a -somewhat- plausible scenario of how a Greater Germany could have survived to the present while making as few alterations to the course of actual history as reasonably possible.
By Greater Germany I mean present Germany+ Austria, the Czech Republic and Poland. In OTL this is of course a monstruous and unreal construct; Eastern and Central Poland particularly stand out as having never really belonged to any Greater German state, not even during Nazi occupation. The rest of the lands however have been parts either of the HRE and/or of Prussia at one time and had significant German populations.
My basic premise is that the PODs are mainly demographical, rather then political and so history largely goes on as in OTL but with a different ethnic makeup of the lands mentioned. Present Czech Republic and Poland would gradually be populated by German speaking people and unlike in OTL they would not be expulsed or assimilated, but would remain to become dominant.
Pre-modern demographics were extremely volatile: a plague or a famine could wipe out large percentages of the population and for many hundred years most European countries had very fluctuating populations. So what if chance had made the fluctuations a little different?
Here are the alterations to OTL that should work together to reach the above scenario:
-> The Early Medieval ethnic makeup of the mentioned lands settles along somewhat different boundaries mainly due to chance during the Migration period; Germanic tribes extend all the way to the Oder and maybe beyond into Silesia instead of just to the Elbe; greater parts of Austria are settled earlier
-> During the Ostsiedlung a greater number of German speaking people settle in what is now Poland, reaching even the Vistula valley. The reasons for this would be mainly demographical: a more fortunate evolution of the German lands ex. less plagues and conversely a more unfortunate evolution of Poland, which in OTL largely escaped the Black Death- all in all more people to go, more people to replace. Some lands become parts of the HRE and of the Teutonic Order, mostly those which did in OTL as well; the key difference will be that unlike in OTL Poland would be settled by an important German speaking minority which would maintain its cultural identity in the centuries to come. This would be not unlike Hungary or the Czech Republic where Germans have also settled in large numbers, sometimes at the very invitation of the local rulers. The Transylvanian Saxons for instance have been loyal servants of the Hungarian Kingdom for centuries, but they did not assimilate and at times made up as high as 15% of Transylvanian population.
-> 1815; half a millenia later. The population of Congress Poland is sth. like 30% German; even less would do. This and better political manouvering on behalf of Prussia and Austria results in the 1795 Third Partition boundaries being restored instead of Poland being awarded to Russia. Throughout the 19th century the Polish lands evolve under a German rule and thanks to extensive campaigns of Germanisation and resettlement, especially in Prussia, the balance ultimately tips in favour of the German ethnicity. The same thing happens in the Czech Republic only in a much more pronounced way; after all Bohemia and Moravia was part of the HRE for centuries and DID in fact have a 30% large German minority. Not-so-massive historical accidents could have resulted in Germans becoming the majority in these lands, which is what I presume here.
Finally a quick summary of the XXth century> after WW1 Austria and Bohemia would become independent republics, after WW2 what is now Poland would become what Eastern Germany was in OTL and eventually after the Fall of Communism they would all peacefully reunite.
Obviously this is just an idea with many holes, not a functional scenario; I am mostly curious whether you find the 'Big Picture' plausible at all, regardless the many small ASBs.