A handful of EMUs to work services north of Newcastle could allow for an hourly Newcastle to Berwick stopper to remove some of the smaller stops from the InterCity services (Acklington and Chathill would be served by alternate trains due to their low usage). This could then connect with ScotRail's Berwick to service if and when they finally get around to running it.
One of the fasts could then run non-stop in each hour, with the second calling at Berwick only, and the third running semi-fast and calling at Dunbar, Berwick and Alnmouth. There could be another short Newcastle - Morpeth working if demand requires it, if not then Morpeth could be added to the semi-fast too to give two trains an hour into Newcastle.
Ideally the re-opened line to Ashington would be electrified too, which would allow a more sizeable allocation of units (say eight or ten) to be based at Heaton, with the freed-up DMUs operating a Newcastle to Middlesbrough via Durham service and improving capacity on other routes.