The Danes and Prussians civs are the most suitable for the Baltic DLC. The Danes civ would be a naval focused civ - maybe they would have a unique Dock. The Prussians civ would be a civ focused on infantry and industry - plus they would have a unique Age Up (similar to the USA civ) that would represent the path the Prussians took to unify Germany.
Poles are more suited to Wild Fields DLC along with Tatars and Ukrainians (or Ruthenians) civs. Along with such a DLC, an update for the Russians civ could take place. Then Russians, Poles and Ukrainians civs would share a brand new Eastern European Architecture Set and Eastern European units - which would replace the default units like unique Ottomans units.