I think the biggest game changer would the section from Humber College to Woodbine GO.
We need to seriously prioritize building this extension. Now. If nothing else, because of how cheap it is: an elevated guideway along a straight-shot path through low-density industrial, with perhaps 1 new station at Rexdale. Only complication being the approach at Woodbine GO itself.
Currently, to get from Union to Humber College, a student must take Line 1 all the way up to Finch and transfer to Line 6. Thats a very slow journey — a fact for which Line 1 shoulders much of the blame.
With the Line 6 extension, the same student could instead take the UP to the new Woodbine station and transfer to the extension. A blazing-fast 2-stop journey later, and you’re at Humber.
The time savings would genuinely be life-changing for a lot of people.