I would imagine, eventually, MLB expansion is going to get a bit complicated. You aren't just adding an MLB team, you have to add 4 relevant MiLB teams too (not including rookie ball fodder). Probably wouldn't be hard to get A/A+ facilities...but the A/AA levels isn't quite as simple as those facilities take huge investments. Once you then find legit options to place them that can support them, at this point, you are probably looking at areas with nearby MLB teams. Then you have to worry about encroaching on an MLB teams market or even another MiLB teams market. Small Markets probably don't want a MiLB team they own near them, let alone a team they don't have any ownership interest in.
I am a bit skeptical of international expansion. Latin America is probably incredibly intriguing to MLB (especially after the WBC excitement/interest), but lets be honest, a lot of those countries are incredibly sketchy. Mexico isn't exactly a place that is considered safe or a place one should go to outside of a resort's walls. Random international series, cool...81 home games, a lot of risk of really really bad PR.