Moving on from MAD, I would have thought FRA minimum 2 x daily ought to be top of the wishlist, and I'm surprised that NCL hasn't got that yet - even once daily. If not FRA, then MUC surely.
One of the problems the United New York service had was the lack of Star Alliance frequent flyers, with the regions big spenders tied into the other alliances. Slots is usually quoted as an issue for FRA, but this is probably also an issue.
Anyway, we are pretty well served for hubs if you're going east.