Alitalia isn't going anywhere, the Italian government's threats are empty, and AA knows it.

However, if AZ were to hypothetically cease to exist with no replacement, then you could possibly see AA return to Milan from Miami and New York City. Alitalia has only two profitable destinations in North America: Miami and New York City. Boston, Chicago, and Toronto perform so poorly during the winter months, that the money they make in the summer doesn't even make up it. Alitalia offers poor connections via Milan, and relies on strong business traffic from Milan to NYC and Miami, as well as Rome to NYC, to generate profits on those routes.

As for where the aircraft would come from...it is true that AA is very short on long-haul capacity, but with clever aircraft scheduling, especially increasing the utlization of 772s (which are heavily under-utilized due to extra-long ground times in South American and London), AA could free up about three extra 763s and 772s each with no route cuts. BOS-CDG is likely a goner in due time, so that will also free-up aircraft. BOS-LHR' daylight flight (huge waste of aircraft, especially because of long ground time) is also likely going to be discontinued, replaced with a 2nd MIA-LHR flight that will have a quick turn around and effectively free up "half" of a 772. ORD-PEK can be added needing only one new aircraft, again, with clever scheduling and reshuffling of some schedules. You'll be surprised at what AA will be able to add in the next 2 years by becoming a little more flexible on schedules. Though you will still see some things not happen. For example, it is looking very unlikely that O'Hare-Buenos Aires will become reality.