My prediction? Zero chance of the Arena site being returned to railway use.
I would tend to agree because I am coming around to the opinion that two arenas in Manchester will work and Manchester already has four large venues used for music in the city: the arena, Heaton Park, Old Trafford cricket ground and the CoM stadium (three of which are on tram lines outside the city centre). People do travel big distances for a gig and most bands on international tours won’t play every city in the UK, therefore a fifth arena in Manchester will probably see the arenas in Liverpool, Leeds & Sheffield among its primary competition on the basis that people are willing to travel.
The fact that the new arena is out of the city centre is not an issue either. In addition to the three existing venues outside the city centre, we then have the proper Old Trafford itself, MediaCity and the Trafford Centre, where people travel big distances to reach.
Most cities have their arenas located outside the city centre and I see no reason for this one not to work.
In conclusion, Victoria won’t be getting a chance of additional through platforms, unless there happens to be a way of engineering below the arena. On platform 6, what exactly is the other side of the wall? Is it literally the arena? Or does the arena itself sit above that space and something else occupies the area where a possible P7 & P8 could reside?
One potential is for east facing bay platforms where the multi-story car park is (which runs a decent length of the through platforms) or remodelling the existing P1 & P2 to create additional bays, but that depends on how much need there is for more terminating capacity from the east side of the city and what it would be used for.