People fly across the Atlantic everyday in narrow bodies with American Delta United Aer Lingus and Norwegian so why not Monarch? And not everyone from the regions likes using a hub airport.
If they did Florida the Caribbean and Asia then I can't see them using the MAXs they'd have to get A330s or 787s.
Monarch are a different airline now so I'd imagine they'd be looking at being a long haul LCC rather than charter.