Part of the problem with bringing back 747-200combis is that while flying mixed pax operations works well on some routes, it's not so good on others. LHR and FRA for example both have high cargo demand and high pax traffic. On the other hand a few years ago the Brussels route was running crammed with freight but almost empty for passenger traffic, so combis aren't great for routes like that. Also AC's shortage of capacity in the coming months will be due to new 767 service to many new cities, so cargo won't be such a big deal for them on those routes, they'll just need 767's to develop the passenger traffic and 747combis aren't good stand-ins for routes that really need 767's.
As far as the need for cargo capacity, I have definitely heard that cargo is running very full, but would it warrant freighters? 5 years ago the all freight operations were losing tonnes of money so they got rid of the fleet of all freight aircraft, so would there be enough traffic to justify the jump from the current backlog to what could be an overcapacity situation?
As far as to where freighters would fly for AC, I don't think there's any doubt YYZ and YVR would be the primary freight warehouses and operations points. YUL would see the freighters sometimes because of its maintenance base, but probably most of the time YUL freight would just be fed to domestic widebodies on the YYZ-YUL route from the YYZ freight terminal.
As far as 747-200combis going to all freighter aircraft, that wouldn't surprise me at all. But that's in part because they have the aircraft already. I have a hard time seeing AC ordering any new 747-400F aircraft at a time that they're looking very closely at replacing their current 747 passenger aircraft. Also, with about other 200 aircraft ranging from Dash8's upward being looked at to replace, I wonder if they'd be ready to buy freighters before the rest of the fleet is replace. I doubt we'll see any orders for 747F's for a few years yet.
One last thing. I would like to see someone else other than AC doing the all cargo operations. Not that I wouldn't like to see AC freighters return, but I would like to see Kelowna Flightcraft, ICC Cargo, Winnport, or whoever else take a go at all-cargo operations for a while before AC goes in.