How feasible would it be to four track the route between Cornbrook and Deansgate along the old viaduct to Manchester Central and build a couple of terminal platforms.
If its going to be a terminus again perhaps Deansgate ought to be renamed Manchester Central?
Not very because you'd need to strengthen the viaduct further, build new platforms at Cornbrook (and therefore remodel Cornbrook junction), refurbish the second bird cage, move all the OHLE on the main viaduct, remodel DGCF further and build more new platforms which would only be used every so often because Deansgate isn't planned to be a terminal for anything but merely providing terminating facilities should they be needed as well as provide more capacity into st. Peters Square for 2CC. It's already getting a third platform there's no need for further building
What is the point? Seriously, Deansgate-Castelfield reflects the area better than Manchester Central does. It's not going to be a terminus again, it will see trams terminate there during the SPS rebuild and in the event of disruption but there's not going to be a dedicated service terminating at DGCF. It'd be like saying that about Timperly, just because trams can terminate there doesn't mean it's going to be a terminus like Altrincham it's just to improve reliably and flexibility.