On the plus side, if demand does indeed remain this strong and increases with the new stations, making a case for funding further upgrades should become much easier. Sadly though I think electrification is very far off. Getting the fares fully integrated with Metro would also be a very good start - the current setup is very much a minimal (and in my view poor) effort.
The section alongside Metro could be doubled - the majority of the right of way is wide enough, even if a little tight. The issue is a few bridges would need rebuilding, and also the way Nexus built Northumberland Park station has the island platform there sitting a little too far towards the north side of the cutting, leaving wasted space. A rebuild could solve that though, and maybe create a cross-platform interchange opportunity or at least a better integrated station. Taking over the road bridge to turn it into the new ticket hall would be a good idea, seeing as it has been closed to traffic for as long as I can remember. That said this will be the last section to be doubled, due to the cost of the aforementioned works.
The rest of the route would be easy to dual, and longer-term it would be nice to see stations added in at Seghill, the south of Ashington to serve the Jubilee estate, and maybe eventually Backworth near the level crossing since that area is going to see housing development soon. Assuming Northern's new units are specified for decent acceleration, adding a few extra stops wouldn't increase end-to-end journey times all that much, but would only further boost passenger numbers.
Sadly I don't think the stations are set for anything more than 4 cars from what I've seen, though I'm happy to be corrected here. Lengthening them wouldn't be the hardest task though!
On the rolling stock front, it does strike me as odd that we're seeing some of the TfW 150s go for scrap right now, when Northern could certainly use the extra units. Yes they're old and riddled with problems, but so are Northern's 150s - so a larger number of them would mean a larger pool to draw from, at least until Northern get their new fleet that is currently under tender.
Of course, the other option to boost frequency with the ECML paths being a major restricting factor would be to upgrade the line from Northumberland Park to Ashington so that Metro trains can also use it, either with their "ready for batteries" capability and recharge facilities at Ashington or by way of actual electrification - this would easily allow a 15-minute service, with alternating Northern and Metro services, with Metro going via Gosforth to Pelaw.
The issue with plain service increases on the Northern service, even if extra ECML paths become available, is always going to be the configuration of Benton Junction, which sees northbound services have to cross the southbound track to access the line; grade separation here is just never going to happen.