Commissioner Rob Manfred has spoken more than once about MLB eventually expanding to 32 teams, and a year ago he hinted strongly that the league is itching to add a city outside the U.S, saying, "If we were to expand, I do think a city that makes sense geographically—meaning in terms of realistic travel distances—and is outside of the 48 contiguous states would be a positive choice for us in terms of growing the game."
While Montreal—which had the Expos from 1969 to 2004—certainly figures to be in the mix, Vancouver is another plausible destination in Canada, and Manfred has spoken of Mexico as well, with Mexico City and Monterrey the most likely options. Puerto Rico, though technically a U.S. territory, has a rich baseball history as well as a venue (San Juan's Hiram Bithorn Stadium) that has been used occasionally to host MLB games.
While a North American addition (or two) makes the most sense from a travel standpoint, the recent World Baseball Classic served as a reminder that baseball should continue to focus on broadening its horizons. Why not consider a team in in Europe—Italy and the Netherlands both have professional leagues already—or in South Korea, where the fans are as exuberant as any in the world, and where games played in the evening would be airing in the States in the morning? Breakfast baseball is too a wonderful thing to limit to every four years.