Belfast - North America is a predominantly outbound market, and Jet Blue have zero brand recognition in Northern Ireland. They will struggle to make the route work without significant investment.
They would be more likely to launch a Dublin, Edinburgh and Amsterdam, which have strong inbound, year round flows.
Norwegian had low brand recognition on both sides of the BFS - North America market, and with unknown secondary airport choices and no feed, meant that the route was going to fail from day one.
The only semi semi viable options are United returning, or a DL/VS 752 operation. Anyone else will only work with a huge incentive guarantee.