Expansion is tricky, because you've got to assume it's highly likely a AAA city is getting called up, so to speak. So then you have 2 AA teams moving up to AAA, and so on.
Portland would need a retractable/permanent roof
Vegas has the A's
Charlotte is current AAA
Montreal?
San Juan is a non-starter, imo
Nowhere in Mexico is not going to happen (safety/infrastructure/elevation)
Nashville would work
I want to say New Orleans, but is that a 'major' city anymore? And it would have the same weather issues as Portland/Tampa Bay
Salt Lake City...wouldn't that just be another Coors Field situation?
As for AA moving up to AAA, Richmond, Hartford, and Reading PA have stronger attendance than most AAA teams. And A+ Dayton has great attendance as well.