I think if anyone comes up with a new service to Heathrow from Belfast International then it has a very good chance of stealing pax who want to go to Heathrow, but don't want to go to BHD airport (hence are forced on to easy), or go to LHR from BHD, but really want to go to BFS.
Also, as bmi's LHRBHD loads are always full from what I hear, extra capacity would probably be snapped up straight away to Heathrow for point-point as well as connecting traffic.....
I still think bmi would have the upper hand though from City airport as you mention the fares would be the battle ground
but as Jeremy mentions it will be like an airline like no other - maybe it will be a flying circus with bouncy castles......