I'm sure that once DL opens its new Terminal A in BOS, it will add back LGW. Of course, it may also try to restore the codeshare relationship with VS that they discontinued a few years ago.
Hopefully, they will be please with the coming summer's BOSFCO results and make that year round, like they just did with CVG.
Until Terminal A is opened up in BOS though, I expect they will probably lay low in BOS, as Terminal C is so congested, and if you try to go through security during peak times, you can count on a good 45 minute wait!