The current Germans are basically the Holy Roman Empire, no Austrian Empire. In my opinion, the current German civ should receive changes (mainly visual, e.g. the flag, new Home City and the ruler) and be renamed the Holy Roman Empire. It would be very nice if in this civ similar mechanics were adapted to the Federal States from USA civ - “Electors of the Holy Roman Empire”. This mechanics could eventually represent not only Germany, but also the entire HRE (because HRE covered a huge area inhabited not only by broad-minded Germans but also by numerous French, Italian and Slavic nations). Several “Electors” for each Age Up could, in a very simple way, introduce players to the ethnic and cultural richness of the Holy Roman Empire. You could also add some Unique Units unique to each “Electors”.
As for Prussia and Austro-Hungary it should be brand new civs with completely new content (some things can be shared for each Germanic civ).
I invite you to read my topic regarding a potential European DLC - I have included my proposals for new European civs (between Prussia and Austro-Hungary) and changes to Germans civ as well as many more things 