I was always really surprised how well DUS did from BHX and BRS. It is not an obvious one and attracts little or no leisure traffic. But DUS covers a big industrial area in Germany, Dusseldorf, Cologne, Bonn, Essen and some other smaller but heavily industrial centres in the Rhur Valley. DUS airport has its own rail station on the national network which makes rapid connections easy.
DUS was regularly BACX best German route and the one Fly Be really kept going as a priority, it was the only one that kept its night stopping aircraft. DUS makes its cash on the SUITS not the SHELL SUITS. Whether there are enough SUITS in the CWL catchment to make it viable who knows. Probably won't find out for a couple of years yet though ! Best bet would be for WAG to talk nicley to one of the Team Lufthansa companies. Think it's Eurowings at DUS but could be wrong. DUS is only a bit further than AMS so wouldn't be too uncomfortable in a Q400 if Fly Be started a base.
Don't hold breath for any new routes from anyone to anywhere anytime soon !