The only reason there is such a frequent service to Heworth, is because of the lack of a railway into Washington and the remnants of pre-regulation services which obviously make GNE money.

If a station was built in Washington (it would need to be upto an interchange standard), then the chances are buses will end up going there just as frequently OR GNE would up the ante and provide quicker, faster buses to Newcastle in competition.

Looking at the bigger picture, if the Nexus QCS came into fruitition, then they probably would integrate all services and link them to the metro at various points.

A station at Nissan would be a given - probably at the Sulgrave crossing. Im not sure about anything further North, but there would need to be something near Peel Retail Park/Barmston and on the site of the old Washington Station.
If it continued south over the viaduct to Sunderland, Im not sure where the next station before South Hylton would be due to the lack of housing and employment near the line.