I think it's unlikely that AC
or any other Canadian carrier will start any 5th freedom services via intermediate points in the USA using their own aircraft. It's more likely that they will code-share on another carrier's services on the sector between the US and the 3rd country.
If not mistaken, if a carrier operated their own aircraft between Canada and any country with an intermediate stop in the US, with 5th freedom rights beyond the US, all through passengers to/from Canada would have to deplane at the US stop and clear security and then reboard for the 2nd sector. That would result in a very unattractive product for passengers travelling to/from Canada. Frequent long lines at security checks at major US airports would also negatively affect on-time performance as the flight obviously couldn't depart until all the through passengers to/from Canada had cleared security and reboarded. It would also mean that passengers of many nationalities travelling to/from Canada would require a US transit visa with all the related expense and red tape simply due to the intermediate stop in the US. These days, most foreigners do everything possible to avoid connecting in the US for that reason, never mind the security hassles and delays.
The ability to serve certain countries beyond the US on a code-share basis may make sense where the traffic doesn't justify nonstop service to/from Canada, but I doubt there will be much if any new direct service via the US using the expanded 5th freedom provisions of the Canada-USA Open Skies agreement.