I'm surprised how people think the reopening of Central would only have marginal benefits and more so that it was the right decision to close it.
Say if Central was reopened, then the next two obvious projects within the vicinity would be the repairing of the GN Castlefield Viaduct to allow trains out of Central to use this and carry on to Cornbrook and a junction there. They would then scrap Metrolink through Chorlton and restore the South District line to heavy rail, rebuild Heaton Mersey Viaduct and double track from Cheadle Heath to Hazel Grove.
With those three projects complete, all CLC Liverpool trains could then run into Central and either terminate there or reverse out and take the restored South District line for Sheffield-East Midlands/Cleethorpes services. This would mean all Liverpool CLC traffic being removed from the Castlefield/Oxford Road corridor and giving this a massive capacity boost. It would also free up a path between Manchester and the Airport/Stockport, with the diversion of the Cleethorpes service. The scrapping of Metrolink services through south Manchester could be compensated for by running a similar shuttle service to what existed before the initial closure, with a reasonably frequent stopping service between Central and Cheadle Heath (reopened) stations.
Finally, in conjunction with Hope Valley capacity improvements, this arrangement would allow EMT to run an hourly service from St Pancras to Manchester Central at a cheaper price compared to Virgin, without the issue of revenue abstraction; since their trains wouldn't share any of the tracks that the Virgins use. It would provide a genuine alternative for getting to London from Manchester.