Either a Holy Roman Empire that never breaks up after Charlemagne, in which he has only one son, and is very effective at teaching him to be a great leader. Thus, no animosity between the Franks and Saxons, as the Germanics are united. Eventually they conquer OTL Prussia (1871), Northern Poland, the Baltic Republic's, the Netherlands Scnadinavia. Eventually Italy is conquered, but the Caliphate holds Spain, and Sardinia. This leads the Pope to re-declare the West Roman Empire, causing a lot of friction with Byzantium, as they view themselves as the TRUE Roman Empire now after the West fell.
An 1848 German Empire, that absorbs Austria-Tyrol, Schleiswig-Holstein, Elsace-Lorraine, Eastern France, and allies with Russia during WW1, because they give Russia a free hand in the Balkans, resulting in German victory. The other one I would like to consider, is when Prussia first revolted against the Poles, to have them in Personal Union with Russia, and conquer OTL 1914 Germany-Poland 1938-and Russia 1914 with Alaska. Do not know how I would do that of course.
For the furthest extent of a Germanic kingdom, I would say 1914 Germany, West Poland, Austria-Tyrol, East and North France, Netherlands, Belgium, Luxembourg, Lichenstein, Kievian Rus, Archangelsk, Karelia Peninsula, Scandinavis, and North Italy. This would be the uber German Empire.