San Juan has the same problem Mexico City has: It’s a poor area relative to the major U.S. markets, and wouldn’t be able to generate enough revenue to support the salaries needed to field a competitive major league roster.
The Latin city which would put all the others to shame in supporting its team would be Havana if those damned Castro brothers would die. Okay, some other things would have to fall into place as well and this a far-into-the-future candidate but I don't think that any Western Hemisphere city would have more potential once Cuba gets out from under its political yoke.