If this whole low-cost long haul thing actually kicks off - there's a real chance it won't - then I'd say BFS would be lucky if any legacy airlines wish to operate transatlantic flights to NI. Just look at what happened when easyJet came into town - BFS lost BA, KLM (and even their BHD services are a significant reduction of what they were from BFS in the '90s).
The A321LR will be brilliant for the likes of Dublin and other transatlantic hubs, but for BFS we won't see them unless Norwegian or other LCCs are operating them to the likes of PVD.
We can always live in hope that one of the 'big three' return to BFS, but Norwegian has pretty much put the nail in the coffin for a proper NYC or BOS service.