I think an hourly Berwick-Newcastle EMU stopper would be a reasonable idea, to take up passengers from Morpeth and Alnmouth and replace the Morpeth-Metrocentre service. An EMU would have higher acceleration and a higher top speed than the current DMU service, so wouldn't get in the way of the fast anglo-scottish services as much as an equivalent DMU operated service would, plus there are turnback sidings at Berwick (including an electrified loop).
In days gone, there used to be such as service that used the south facing bay at Berwick - a picture I've seen shows a 143 in the TWPTE colours stabled there. This could be a good idea, although as has been said people from Berwick would choose the faster EC/XC train, unless there was an incentive (such as a cheaper fare, similar to what happened when the Javelins were introduced).
The Morpeth-Metrocentre shuttle could then be cut back to Newcastle-Metrocentre, and increased to half hourly (it is currently hourly right? It's so long since I've been back home to the north east that I can't remember) to give a 15 minute Newcastle-Metrocentre frequency.
There is 2 tph Morpeth - MetroCentre, 1tph Middlesbrough - Hexham and 1tph Newcastle -Carlisle, which effectively means one every 15 minutes anyway. However you normally find the Carlisle and Middlesbrough services are packed with people going there, especially at weekends and school holidays, and the Newcastle/Morpeth-Metrocentre services are fairly lightly loaded.
I don't much fancy the idea of the Metro running through to Metrocentre via the current heavy rail route (not that it'll happen now anyway), it would interfere with the heavy rail services too much. I'd be interested to know how it was envisaged that the Metro would link into the Tyne Valley Line when this plan was mooted? Extending the Metro from St. James into Newcastle's west end on the north side of the river would be a much more sensible idea.
One of the many plans I've been told about was to run Metros (or even heavy rail again) along Scotswood Rd like it used to, use the bridge that is still there to get to Blaydon and reverse to the MetroCentre. New stations would be built at Scotswood and Elswick, and would mean that Blaydon and Dunston get more services per day. The train would then get back to either Central via Gateshead and the ECML. How it could link back into the Metro system from there would be tricky. I wouldn't think that any extension of the Metro would go past Blaydon (perhaps as far as Wylam) due to it being outside Nexus's remit.
It would be good to get the Metro into the West End of Newcastle but tunnelling would be too costly. Instead it was envisaged under Project Orpehus that tram-trains would be able to run on the surface from the west end and enter a tunnel at St James to continue the journey from there.
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EDIT - there is 1tph Morpeth - MetroCentre not 2, but there is the Newcastle - MetroCentre shuttle which effectively means 4tph Newcastle - MetroCentre, although the timings do change through the day.