Germanize the Slavs some way. Such as a German Cyril and Methodius bring Latin alphabet, Slavs adopt Catholicism. German settlers arrive in great numbers in the 9th century, mostly as burghers. The Mongols devastate the rural Slavic population, making room for more Germans later on. Think Volga Germans or Baltic Germans but on a much larger scale. As the German Demetrius von Don defeats the Mongols and unites the principalities, the dominant German culture steadily Germanizes the remaining Slavs, somewhat like the Prussians IOTL. The butterflies are of course immense, you have a huge German state in the East, and any Slavic states, such as a rump Poland, or Kiev are quickly overwhelmed and assimilated. From here on, it just takes the collapse of the HRE for this state to assume leadership over all Germans. The Kaiser of All Germans. Hm, I like the ring to it. This is of course likely the biggest Germanwank ever conceived.