The HRE's stability is, in many ways, due to the (mostly) ahistorical German monoculture in its boundaries. Many idea for splits have been thrown about, but in a medieval context, I only really find one model convincing - the one that splits the cultures up along the lines of the stem duchies.
Therefore, we would probably need 5 new cultures (four German and one French):
-Bairisch - that covers Bavaria and Austria
-Schwäbisch - covering Swabia, German Switzerland and Alsace
-Fränkisch - covering Franconia and the Upper Rhineland all the way to Cologne
-Sächsisch - covering Northern Germany
-Lorrain - covering Lorraine, Bar, Montbeliard and other French-speaking parts of the HRE.
I know that often dukes and counts from other tribes were put in charge in many regions of Germany, but I thing we can mostly disregard any discrepancies in culture for balance's sake (like PDS made Saladin Egyptian rather than Kurdish).
Therefore, we would probably need 5 new cultures (four German and one French):
-Bairisch - that covers Bavaria and Austria
-Schwäbisch - covering Swabia, German Switzerland and Alsace
-Fränkisch - covering Franconia and the Upper Rhineland all the way to Cologne
-Sächsisch - covering Northern Germany
-Lorrain - covering Lorraine, Bar, Montbeliard and other French-speaking parts of the HRE.
I know that often dukes and counts from other tribes were put in charge in many regions of Germany, but I thing we can mostly disregard any discrepancies in culture for balance's sake (like PDS made Saladin Egyptian rather than Kurdish).