Edit:
I know that there were many hanseatic cities in the Rhineland, Saxony and English Channel nodes, too. Yet i guess no one wants to see all of Northern Germany belonging to a single league. So maybe you have to be Saxon, Hanoverian, Rhinelandish, Prussian or Frisian to be able to join. Another prerequisite could be a coastal port or a location at an important river (Rhine, Oder, Maas, Elbe).
Something like this? Different "circles":
http://pages.uoregon.edu/kimball/grd.Hanse.htm
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