I would be surprised to see Alitalia return to Montreal with their own aircraft. I could see a codeshare with Air France - Skyteam. Alitalia would have to build up its Italian hubs and attract connecting traffic from Canada through Italy.
Traffic between Italy and Canada matured years ago. Italians living in Montreal today no longer have the ties they used to have to Italy. In addition to Italy, they are also now vacationing in other places - Mexico, Florida, Switzerland for example. The Italian market was heavily VFR (visiting friends and relatives). That has dropped off.
Years ago, Alitalia provided 747 service to Montreal. They also provided connections in Rome to the Middle East and Africa. These connecting passengers now transit other European/North African airports.
The local Italian community was "outraged" when Alitalia downguaged to an MD-11...until Alitalia's PR department said it was a great aircraft. The Italian community was "outraged" when Alitalia pulled out of Montreal. Even a Canadian parliamentarian voiced his opinion - forgetting that he represented Canada and that Canadian Airlines still served the market at the time. Alitalia downgraded its Toronto service to seasonal - although they continue their codeshare with Air Canada (until the Skyteam agreement begins).
Air Transat continues to serve the Montreal - Italy charter market. The tour operator caters to tourists and VFR traffic.
Air Canada did have this route on their wish list but, like most other global carriers, these wish lists have been moved to a lower priority. Keep in mind that Air Canada still has a code-share agreement with Alitalia and, if the traffic materializes, one of these carriers will operate the route. From Toronto, Air Canada currently has only four flights per week each to Rome and Milan.
In the mean time, scheduled passengers to Italy are connecting in London, Frankfurt, Paris, Zurich, and Toronto.