If we are talking about fantasy, I agree a link between Sheppard West and Sheppard-Yonge should be built but not as part of Line 4. Instead they could convert Line 1 into a giant loop along the new Sheppard link isolating delays between the two directions. Line 4 will remain between Yonge and Don Mills. Sheppard West to VMC can become the Vaughan Line. Sheppard-Yonge to Richmond Hill Centre (Yonge extension) could be the North Yonge Line. Yes, it means people from Finch to downtown would need to transfer but it will increase capacity on a new Line 1 loop. On Weekends and late night, the could revert to the current structure with the extensions.That may work in theory but the best best of these options is to extend Line 4 west. The main reason would he to cut back the travel time of trains from Line 1 to get to Wilson yard. This is a low priority as the Yonge extension if/when built includes some track north of the terminal to store trains to be used for start of service.