I have a feeling that if it's a full service carrier, it'll be with Delta Air Lines using a 757. They fly to Edinburgh and are starting seasonal flights to Glasgow, so they seem to be expanding past Manchester/London, though only during the summer months.
I can't see American Airlines doing it, given they've just pulled out of Birmingham.
I'd say if Aer Lingus had remained at BFS, they might have taken over using a 757, though I doubt that BHD has the runway length to handle a 757 for a short sector, never mind transatlantic.
I do hope, even though it's extremely unlikely, that Virgin Atlantic expand out of Belfast as they're doing out of Manchester. Basing an A330/B788 and flying to a mix of Delta hubs and leisure destinations such as Atlanta, Boston, Las Vegas, New York, Orlando 2 or 3 times a week would be a huge leap forward for connectivity with the US, even if it was just during the summer months. But that'll never happen - honestly I'm surprised VA have seen any success in Belfast.
If Norwegian start flying, either to JFK/EWR or somewhere like Stewart Intl, I can't see it being very successful, given the lack of connecting passengers. A seasonal 738 service might be sustainable if flown 3-4 times a week, but certainly not daily.
Whoever takes over the New York service, I'd be very surprised if it was more than 5 weekly flights during the summer and 3 during the winter.
On another note, if Belfast does ever get another Canadian service, I'd be very surprised if it wasn't at Westjet 737 with a stop-over in Halifax/St Johns.