So like I've said many times before, NYCC (including the Yorkmills node) is a fairly stable office market, with total vacancy between 6-8% (downtown is closer to 2% to give some perspective ... which is ridiculously low ... but the GTA {905 && rest of the 416} is closer to 11-13%) ... availability is closer to 10% ... this includes sublease space. There have been some tenants that have relocated downtown but a few new leases have been signed.
If you want to know more I suggest you read various office market reports, this one is probably the best as it focuses on suburban 416 and the 905:
https://www.avisonyoung.ca/documents/56635302/56647756/Toronto+North+East+Office+Market+Report+(Q4+2019)?_ga=2.76439699.74063563.1581828371-403501699.1581828371
I don't imagine much will change, if anything I think vacancies will continue to have a slight upward pressure as you see a few more relocations to downtown and potentially VCC. But, as time has proven, they do tend to be slowly backfilled.
So with all that said, I doubt there will be any new office construction for the foreseeable future (less that of the `forced`variety as I'll call it) ... the build costs just don't justify it given the rental rates (compared to say downtown). That's just half the problem, the other is, it's really rare you see a large bulk deals in NYCC, most are in the 25<->50K square/foot range i.e. building up enough demand to fill up a new decent size tower (say 20 floors) will just take a very long time.
Given that, it's likely there will be NO new office construction, I can imagine most of the remaining suitable plots of lands will have condominiums built on them, with maybe the only exception being the N/W corner of Sheppard and Yonge.
While it's not likely the 2nd downtown some imagined decades ago, it's still a mixed use community with over 10 million square feet of office space ... that's nothing to sneeze at, in fact, I suspect this is more of what you'll see in the GTA outside downtown going forward (using downtown Markham, VCC as examples ... both of which, likely won't have the same amount of office space as there is NYCC for years if ever).