Is the northern part more or less free from future construction considerations ?
From Bloor to College, which I would take to be the 'northern part', there are 2 university developments proposed, the planetarium site and the redeveloped Northrop Frye.
There is also the Museum Station second exit which will literally be in Queen's Park.
None of these would necessarily interfere w/the existing roadway, as the buildings are set well back from the road..
Additionally the majority of any work north of College, the for the park itself, would be on the west side of Queen's Park Crescent, and south of Hoskin, so I think the conflicts, if any, would be minimal.
Though, removing QPC west, may mean adding one lane on the east side, which could prove to be controversial with mature tree removals, and that may have more conflicts.
The thing about starting at that end is that I think that would be very contingent on narrowing Avenue Road north of Bloor by one lane each way in order to manage the capacity reduction south of Bloor.
I'm all in favour of that, but I'm not sure how quickly Transportation is prepared to move.
I would have thought it made more sense to start at the south end in coordination with the Ontario Line......but perhaps that is seen as too convoluted.