Given BA pulled the MAN>JFK route quite a few years ago, would be surprised to see them back on it. Was a nice route on the B767 (took it once, enjoyed the 2x3x2 WT cabin, especially with nobody beside me!)
I don't think a JFK rotation with a B788 would work as the aircraft would have to be cycled through LHR for maintenance and rather then schedule random MAN>LHR rotations, carriers tend to prefer to cycle aircraft through the outstation in these circumstances. EWR is only B789's I think, and I don't think an F cabin would be required on that route (I maybe wrong!)
Lastly, as noted above, AA benefited from a same Terminal transfer and obviously couldn't make it work, so not sure BA could get the transfer pax to make it viable if people have to transfer terminals.
Pilot37