Brussels Airlines scrapped BRU-BHD earlier this year. KLM (Skyteam) and BA (Oneworld) are the only alliance links.
Both BHD and BFS have been trying to get Lufthansa/Eurowings for years - however the inability of SN to sustain their Belfast operations doesn't bode well for another Lufthansa Group airline coming to Belfast anytime soon.
I also can't see IAG entering the long haul market from Belfast for a number of reasons. The runway at BHD is too short for transatlantic operations, and they wouldn't split operations between BHD and BFS. IAG will enhance their offering from DUB, LGW, BCN in order to compete against Norwegian, but they won't start operations from regional airports.