I think expansion is the only way the MLB grows.
By and large, the number of games an MLB team plays kinda makes "appointment television" and travelling to away games prohibitive for fans (outside of diehard superfans). The real draw with baseball is that you can take the day off work and go down to the stadium and hang out for a few hours and take in the experience.
Also, at least in my opinion, the in person experience for the MLB (like the NHL) is heads and shoulders above watching it on television. I can't say the same for the NFL or the NBA.
I think Nashville is a growing town and the fan support for the Preds will give it a better look for relocations going forward, but it's going to have a hard time drawing an MLB team because of its location. It's in the direct center of the 4 hour nexus of Cincinnati, St. Louis, and Atlanta and I just don't think the population density is there to support 4 teams in that area. Same reason Indianapolis has never really had a team.
Charlotte may be a good option as a Rays relocation destination, but it is still awfully close to Atlanta for a brand new team.
Your kinda "ideal" spots are:
1. Western US (between Seattle, Denver, and Bay Area):
Portland - yes, it's still close to Seattle, but part of Portland's identity is that it is "not Seattle".
Salt Lake City - the location is right, but it's been a hard sell in the past.
Las Vegas - If the Knights and the Raiders do well, this would be a no-brainer. They'll even have a new(ish) stadium ready.
2. International:
Mexico- Mexico City has been tossed around for most of my life, but the travel costs for teams to and from Mexico City are going to be difficult to swallow. The side effects of the drug trade through northern cities like Monterrey are probably going to keep the MLB out.
Canada- Vancouver seems like a no-brainer, but they couldn't support the last pro franchise that came through, neither could Montreal.
Cuba - 2-3 years ago, I would have thought Havana could be a really interesting proposition, but the current administration seems to want to tighten up the recently relaxed restrictions on Cuba, so who knows?
Puerto Rico - (yes, I know it's not technically international) If Congress would do the right thing with Puerto Rico's finances (even before the hurricane), San Juan has the population and infrastructure to support a team, but not the finances. Being an American territory, it wouldn't have the same concerns as Mexico or Cuba.