I think I made a thread to that regard some time back. Question is of coruse wether there are enouzgh Germans to settel even more lands at all!
I guess, whats possible is that somehow Poland doesnt consolidate at state while at the same time remaining pagan - thus becoming in effect the same as East-Elbian Germany. Thus it can be christianised over time, and in that process becomes Germanized a swell. The problems just... East-Elbian Germany plus Poland, thats a LOT of land. I cant really see it ALL becoming Germanised in the process of Christianisation. And then also additionally Lithuania...
What may have more potential is the Pannonian Plain. Say an ASB ISOTs all Magyars to Middle Earth before they reach Central Europe (

, that is, the Hungarians dont settle in the Pannonian Plain) and that Charlemagne Carantanian Marches survive. They could in time expand to maybe cover the entire Plain. Of cours,e the problem with THAT is that no Magyars means no rise of Otto I the Great - or, more geenrally spoken, less impetus for the unfication of the German tribal duchies. Its nopt inevbitably that way, but it could be that Germany doesnt even hold together as realm an dnation - though we can make that a third condition next to no Magyars and no fall of teh Carantanian Marches, of course.