Could a 737 Max from a USA carrier with Domestic First class seats like Air Canada not work from Belfast? It'd be smaller than a 757 and they'd not have to worry about Yield from Business Class because it'd be cheaper.
The 737 MAX in that configuration is much more flexible and wouldn't have fleet issues i.e with being used on transcontinental flights where United cancelled several transatlantic flights to keep up with Jetblue and others competition, because it wouldn't be in that Premium heavy configuration.
Yes, I know that Norweigan had this configuration. But they're not a legacy carrier with connections and flights into major international airports, nor do they pull from American mileage fliers.