Baseball SHOULD return to Montreal. No franchise has ever had crappier ownership than the Expos....between Brochu's group and Loria, that would be the death knell for any team.
As we've seen in hockey, CFL football and even pro wrestling, Montreal fans are as passionate as they come about their sports. Hell, even the 5000 regulars who were still coming to the Big O at the end were still making a lot of noise and were totally into it. If Montreal could line up legitimate owners and build a Safeco Field-style ballpark downtown, a franchise could absolutely thrive there.
As to what teams could move, the Rays and Athletics stand out as the obvious candidates. Les Montreal Atletiques?
"It breaks your heart. It is designed to break your heart. The game begins in the spring, when everything else begins again, and it blossoms in the summer, filling the afternoons and evenings, and then as soon as the chill rains come, it stops and leaves you to face the fall alone." --- Bart Giamatti, on baseball
I don't really want to debate his hall of fame merits right now ... but if he gets in, I will wish then that it had happened this year. Wright Thompson at ESPN has a nice column (sports.espn.go.com) up about O'Neil that I would point anyone to.