In case FR want to start more German domestic routes below are the Top 10 from 2015:
Berlin-Munich 1.98 million
Berlin-Frankfurt 1.91 million
Hamburg-Munich 1.81 million
Dusseldorf-Munich 1.55 million
Cologne-Berlin 1.46 million (FR already flies CGN-SXF)
Hamburg-Frankfurt 1.36 million
Stuttgart-Berlin 0.99 million
Stuttgart-Hamburg 0.72 million
Dusseldorf-Hamburg 0.56 million
Cologne-Hamburg 0.39 million