The challenge, if you accept it is to make slavic people present on what is baltic coast of Germany OTL and as deep in land as Thuringia.
1)The slavic people here does not have to be historical wendish confederations (Abodrites, Lusacians, Sorabians, Veleti, etc), but it's all bonus points.
2)Integration into HRE allowed
3)Christianization allowed
4)Integration into non-slavic countries in 2010 allowed but they still have to represent the first population (absolute or relative majority).