BA had stated in the recent past that it did want to return to Northern Ireland, I think in the context of LHR getting the third runway. With this in mind, and the alleged profitability of the current BD Belfast route, I think there's a real possibility we could see BA back in Belfast.
However, currently we have up to nine LHR flights per day I think??? EI - 3 and BD - 6?? So I could see BA wanting to keep the current bmi traffic. They could do this buy hoping all current bmi customers stay with the route, plus pulling the BA codeshare from the EI BFS flights, making up for any shortfall that may happen. Therefore a 3 flights a day operation would in my mind be a little thin.
When bmi operated to both Belfast airports, yields and demand appeared much stronger at BHD, and with less competition on the London routes from BHD, the airbridge and direct business lounge access, I'd bet my money that BA would stay at the City.