No way MLB would go for a 21,000 seat stadium. I think the newish ballpark in uptown Charlotte could be expanded a bit, but I also don't see Charlotte supporting an MLB team for 81 dates.
As someone who loves skyscrapers, that park is one of the greatest parks in the country.
I am curious why you don't think Charlotte could support an MLB team. Charlotte metro has more people than Cleveland, Cincinnati, Pittsburgh, etc. And Charlotte is experiencing ridiculous growth. A solid corporate presence: you've got Bank of America, Lowe's, NASCAR, Duke Energy, etc all in the city limits or close by. Huge airport.
I think the biggest obstacle would be the Braves raising Hell about their "territory." They'd need some cash and someone to show them that it would be a net positive overall for them.
Since MLB wants 8 divisions with 4 teams, imagine an MLB division with the Braves, a Nashville team, a Charlotte team, and Cincinnati. Would be bananas.
Not sure where you'd put the Rays and Marlins in this scenario, but I am convinced they have to stay. Florida is too important of a market. And the Rays have a new ballpark location in Tampa, so there's hope there.
The Athletics are probably going to move. Can't do Montreal because you need to keep the Athletics brand, and that city is too synonymous with the Expos. I think the Nashville Athletics would be perfect.