If they do reinstall the old train shed, it might make the station a bit more difficult to move around... where would the entrance/ticket barriers be installed for instance? The booking hall is very historic but much too small for it's current use.
Also there is quite a big pedestrian flow across what would be the throat of those platforms, with pedestrians heading to all the new canalside development areas, so some sort of bridge or underpass may be necessary.
Personally, instead of reinstating platforms I think it would be good to sympathetically turn the old train shed into a huge new station booking hall, maybe extend the existing subway between the platforms under the forecourt and then up stairs into this new booking hall... but anyway I'm not in charge, so just a crazy idea..