If WN announces PIT-BOS, US will cancel that route with such speed that it will shock and awe.
I don't, however, believe WN would add that route. It's already served and it would be competitive with B6 (something WN always avoids).
WN is going to add mostly flights to Florida and some highly competitive routes. I almost think BOS-PHL is a sure thing.
I predict:
BOS-PHL
BOS-BWI
BOS-MDW
BOS-MCO