BHX-CGN hasn't worked after so many attempts because the aircraft type each time was wrong. Low fares flights have too much capacity for 6 flights per week. This route would suit CRJs or even Flybe DH4s (they maybe too large).
As for the comment about arriving in Cologne late at night. For leisure travelers its a wasted first night in a hotel for a two or three night break. You get out the airport straight to bed! For businessmen better flight times ex BHX would be to overnight the aircraft in CGN and return to BHX early in the morning capturing both early morning meetings in Cologne area and early morning business in Birmingham. Ideally this route would suit two departures morning and evening with small regional jets.
These flight times coupled with competition from up the M42 with timing better suited for leisure passengers will ensure this service wont work ex BHX, if it even gets off the ground. Mark my words!
Daza