If anyone were to pick up the BRS-USA route, I would also bet on Delta or possibly AA doing it
It's very unlikely for the same reason as BHX. It's too close to hundreds more options in London. US tried BHX and left after one season, albeit one following the banking crisis. Continental scaled back to daily as there was no market for two flights. If anyone was going to make Bristol work it would have been Continental and they are moving the Bristol B757 to Heathrow where they expect it to be more profitable.
Delta couldn't make EDI work which I think is a better case than Bristol, which leaves American who almost started NCL-JFK. Given that they now have ATI with BA and IB, I suspect they will be focussing on each others bases in Europe and BRS of course has no BA presence.
I don't see scheduled, legacy US flights returning in the near term I'm afraid. The marketing teams at airlines are always talking to marketing teams at airports, that's their job. I wouldn't read a lot into it, Delta might have looked at Cardiff but how many Delta B757s are parked at Rhoose?