Sacramento would probably be high on the list. I think they have the 20th largest market in the United States. I don't know how that would affect the The A's though because I hear they might move to San Jose. That may be a rumor, not positive on that.
I don't think the SouthEast is a good spot for baseball, I know Atlanta seems like a baseball city but from what I hear the fans in Atlanta are pretty fair weather. The Braves are lucky to have some good baseball minds because I guarantee if they start losing its gonna be 10,000 attendance nights again. Unless Nashville somehow created a dynasty like the Braves I think they would end up being like the Rays and Marlins.
Nascar and college football fans seem pretty loyal and alot of those fans probably consider baseball a sport for Northern Yankees.