They should just prohibit left turns here. While they could fit a left turn lane, the goal is to shrink the intersection to make it safer for pedestrians. Cars which want to continue from King onto the Queensway can take Jameson up to Queen and make the left there. Though if they're turning it into a more normal intersection, they could maybe instead do an advance green for northbound traffic in the afternoon.It's frustrating to me that they are not creating a dedicated left turn lane of some sort for vehicles at King and Roncesvalles, there is space for it and would improve service reliability immensely for the 504 in the evenings. The backups of left turning vehicles from King St can create 3-4 cycle waits to clear the intersection. Either getting cars through the intersection faster or out of the way of the streetcar would make a huge difference.
The plan was to run the 505 to the loop at Dundas and Runnymede. Then a connecting track along Runnymede to St Clair would allow cars at Roncy to reach an extended 512 much faster, as well as providing redundancy as St Clair currently has just the one connection to the carhouses. The 1997 TTC Opportunities For New Streetcar Routes report (the one which resulted in the creation of the 509) recommending protecting for that possibility once there was more residential density along St Clair. Of course, with the size of the flexities, the loop at Runnymede could never manage the volume of streetcars from both the 505 and 512, even if you short-turned half the 505 service at Dundas West station and half the 512 at Gunn's, so a 512 extension would need a new loop at St Clair (presumably somewhere at Jane or Scarlett).What is the proposed route to extend the Dundas West Streetcars north of Bloor?
This scheme would probably also require a redesign of Dundas West station as you'd now have cars going both north and south serving the station. The station isn't really suited to on street boarding so you'd need a third streetcar platform. So now you're looking at a fairly significant investment for relatively minimal (public-facing) service improvement. If a developer was willing to subsidize it then maybe it's possible (the TTC has long maintained that if a developer on St Clair would like to pay to put in the track, they'd run the cars) but otherwise as mentioned the priority is very much elsewhere.