And BHX-Cologne? Once a day, off peak? Surely the logical thing would be an extra frequency on BHX-Dusseldorf especially given that it's an existing route and the train services from DUS airport to Cologne are so good.
It is a new route at CGN, and usually this means that incentives are being coughed up by the airport. A fifth daily rotation to DUS at an off-off peaktime would probably attract few passengers and be more expensive to operate than a new service to Cologne.
Cologne has seen various BHX services over the past decade. It has been served - in that order - by British Airways, Lufthansa/DLT, Augsburg Airways, Duo, HLX and Germanwings in the past. The HLX and Germanwings flights were operated by 150 seaters which were too big for the market, but an 80 seater should do okay. Problem is that Germanwings now has CRJ900s at its disposal. They have begun operating W-patterns from DUS to include thin routesto/from CGN, e.g. WAW or ARN. Germanwings could always try to do BHX with a DUS-based aircraft in order to keep Flybe out of its CGN sandbox.