I think Loganair is the airline to fill the void left by Citywing's closure.
Belfast City twice daily ideally, on Saab and done in conjunction with GLA and potentially EDI rotations. BHD twice daily on a Saab would be a significant increase in capacity, but given the fares that Citywing were collecting for very short trips, I believe that Loganair could stimulate the market and grow the business. EDI as an aside while non operational of late it had enjoyed stronger numbers than GLA in the mid 2000's, circa 2006.
The underlying challenge will be loganair's ability to cost effectively deliver such an operation without basing aircraft and crew on the Island, in which case a daily Scotland -IOM-Belfast -IOM-Scotland rotation would appear more feasable. They certainly could look to lever costs at airports like IOM/BFS/BHD...