There's been a lot of speculation about what planes will fly JFK-LAX/SFO. Maybe 763s (two or three class). Maybe 757s, which is what UA has used on its JFK-LAX/SFO transcons for a decade now since it retired its 762s. Maybe a mix of planes, similar to what we see now on LAX-MIA.
While the three-class transcons have been nice, I wouldn't be surprised to see them disappear in the next couple of years.
As mentioned above, the 763s would be nice, but they're slated to be replaced by the the 787s (if and when they're ever built and delivered).