Over the past few years, Major League Soccer has been growing rapidly, and there is talk of even further expansion. The most likely target is Queens, New York (perhaps as a revival of Pele's Cosmos), but St. Louis, Atlanta, Phoenix, San Diego and Miami have also been mentioned as targets.
However, I'd like to add one other city to MLS' list: El Paso, Texas.
El Paso is one of the largest cities in the country without any top-level sports teams, at 649,121. Only Austin has more people within its city limits.
804,123 people live in the El Paso metro region, not that much in comparison to other American metros with sports teams. However, if you combine the population of the El Paso metro area with Southern New Mexico and Ciudad Juarez across the Rio Grande, the population balloons to over two-and-a-quarter million. That's bigger than the media markets of many one-team cities.
And, mind you, there are people who cross international borders to regularly watch sporting events. The Buffalo Sabres and Bills have substantial fanbases in Southern Ontario.
Furthermore, not only is El Paso-Juarez of sufficient size to support a pro team, it doesn't have any nearby competition. The closest cities with top-level teams are Phoenix and San Antonio. Both are more than 400 miles from El Paso and neither of them have a top-tier soccer team.
Another factor in El Paso's favor is its large Mexican population. With more than 500,000 people of Mexican descent on the American side of El Paso-Juarez, El Paso has one of the ten largest metropolitan areas in the States in terms of Mexican-American population.
There are teams throughout Europe and the Americas that draw primarily from a particular occupation or ethnic group. MLS was successful in that regard with Chivas USA partnering with CD Guadalajara, and I see no reason why they couldn't do the same with El Paso partnering with Cruz Azul or Club América.
Demographic factors aren't the only argument for a team in El Paso: it's not like El Paso is a stranger to professional soccer. The minor-league Patriots have called the city home for over two decades, including for several years in the A-League (the soccer equivalent of the AAA circuit). One of the Patriots' stablemates in the A-League were the Portland Timbers, who of course are now a successful MLS expansion franchise.
MLS in El Paso: it's an idea whose time has finally come.