However, to me it offers the chance for a reasonably timed journey between Manchester and Sheffield
One key word needs added to the end of that phrase;
Victoria! Running through to Sheffield Midland, essential for connections to other routes, trams and buses, would require either a time-consuming reversal at Nunnery Junction or an equally time-consuming diversion via Barnsley. And does not the Midland route already provide reasonably timed journeys between Manchester and Sheffield, on a half-hourly basis?
Waverley line, and have avanti and Scotrail run trains through the line
Waverley extension to Hawick should be seriously considered, but beyond is a non-starter; The distance from there to joining the WCML is over 40 miles through, basically, absolutely nothing. Local traffic cannot justify restoration, and use by main line services would require expensive upgrades as per my post #21.