With fast buses connecting the majority of the regions towns and cities, the only additional service I can think of is something coming from Darlington heading North.
There was the 723 and Arriva have a service which follows a similar route, but apart from the train, there isn't any other option.
Heading North out of Darlington, the service could join the A1 (M) (or serve parts of Newton Aycliffe and then join the motorway) and when it reaches Bowburn, follow the route of the X1 into Durham. It can then complement the X21 to Newcastle.
It will fill a massive gap in the GNE provision and increase presence in an area they are obviously keen on exploiting.
Re the Washington idea above, instead of serving Crowther, would it be better diverting via Ayton and Lambton, turning left uptowards Oxclose and picking up the suggested oute from there?
RE: Service Suggestions