The Expos capitalized on the coincidence of the Montreal Exposition. That was over 50 years ago and few even now know what the theme of the Expo was. Their stadium was an after thought of that event. 20 years later they even tried to leave Canada by referendum. Montreal seems to pride themselves on being more European than the rest of Canada, and that's OK, but I don't see that matching up with a desire for a major league US sports franchise, that's previously been tried and failed.
The baseball season is so long that April and September/October games, in an outdoor stadium, would be unwatchable.