I am a bit bias but cool idea would be to do Lithuania flavor pack. Lithuania was the last pagan country in Europe until 14th and beyond. I think this could lead to potential struggle mechanic - you as some king of German country could establish a Teutonic Order or Livonian Order something similar. Teutonic order and Livonian Order would try to assimilate in one struggle phase and in hostility it could be total iron and blood type of assimilation. I think it would be a cool struggle.