LHR, BRU, and CDG
Do you think that UA might ever add non-stop service on a 757-200 from BOS to some key European capital cities?
TATL flights from BOS to London, Brussels, and Paris might make sense, in particular, due to a) the connectivity to the aforementioned USA cities, b) allowing the existing *A frequent fliers in Boston fly to some key European O&D destinations without backtracking on either end, and c) better utilizing the 752 fleet (instead of doing turns to Florida and Las Vegas)?
(One could also argue that summer-time-only flights from BOS to Shannon, Cork, and Dublin might also be a winner.)