The whole point of 'Picc-Vic' was to allow through-running between suburban services north and south of the city, with a central station in the basement of the Arndale Centre (for which the 'box' was actually built) and freeing up the two main-line stations for long-distance services. Four-tracking or double-decking of the MSJ & A viaducts or flying-junctions for the Ordsall Curve is just pure fantasy - I was struck travelling over the 'corridor' yesterday how close some of the recent high-density developments are to the tracks. The visual intrusion would be monstrous, the disruption during construction intense, whilst some of the listed road over-bridges would be lost. The Manchester Arena won't be leaving Victoria any time soon - it is one of the largest and most successful venues in the country., partly due to its city-centre location and excellent rail links. Revival of Exchange station is now blocked by recent developments, whilst over at Mayfield (now partially demolished) a huge regeneration project is planned.
In the short-term Platforms 15/16 can offer some operational flexibilty, allowing trains to be recessed, and also significantly improve the passenger environment. Platforms 13/14 are downright dangerous - it is only a matter of time before there is a nasty accident due to the hideous overcrowding in the peak. Removal of the platform buildings has helped a bit, but it is miserable place to wait for a train home on a typical cold and wet Manchester night.
In the longer term, 'Picc-Vic' needs to be looked at again, but in conjunction with HS2 and NPR. There is a case for a complete rethink at Piccadilly, with an underground station for high-speed services (favoured by Andy Burnham) to allow through running.