I have often thought about this...As you said it is pie in the sky, so I will keep it going, my suggestions

The idea that makes sense to me is to
1. Re-open the Leamside Line, and use the Stillington Line, would be good for Nissan, and also use it for Frieght Movements between Tyne Dock and Stockton and Northallerton, take some of the frieght off the ECML and it may free up space on an already crowded line...For passengers, sure I read somewhere years ago The Leamside could used for TPX Services

2. Refurbish the Curve at Brockley Whins with Metros going between Sunderland and South Shields and as an alternate oute from South Shields-Pelaw via Fellgate

3. Extend the track from South Hylton up to the Leamside, convert the Northern Section to Metro and run a Circular Service from Park Lane-Leamside Line sand back on the DCL