The way I see it there is a fair chance that they could in fact be looking at some trans atlantic flights now.
The new FTL's that take effect soon seem to have taken long haul into account.
I would say If it were to happen, it could kick off with 737-800's with 150 seats. This would give plenty of room for the pax, and max range.
It would cost very little to try, if it works they can order bigger airframes. If it doesn't, they just reconfigure them to 189 pax and pull them back into european work.