St Louis, Missouri
Population: 2.8 million (Greater St. Louis)
St. Louis's soccer connection goes back to the 60's and 70's, when St. Louis University was one of the most successful college soccer teams, supplying many a player for the NASL's St. Louis Stars. Notable alumni in the modern era include Brian McBride and Tim Ream. While a USL Pro team, Saint Louis FC, is set to debut in 2015, there are no current plans for either NASL or MLS expansion. The stadium that will host St. Louis FC is a bit small for the bigger leagues, but there's no reason why the 60k capacity Busch Stadium can't be shared with an association football team. Like New York's Yankee Stadium, Busch is no stranger to hosting soccer friendlies, selling out for Chelsea vs Manchester City last year. Another stadium, the Edward Jones Dome, hosted a friendly between Bosnia and the Ivory Coast in May 2014.

Tucson, Arizona
Population: 996,544 (metro area)
Better known for its softball, basketball and swim teams, Tucson has seen a growing interest in soccer in recent years. The city is familiar to MLS fans, regularly hosting the Desert Diamond Cup during the MLS pre-season. USL PDL club FC Tucson is involved in organizing the tournament. Founded in 2010, the team plays in the league's Mountain Division, finishing at the top this year for the first time. Tucson was also home to now-defunct USISL side Tucson Amigos. The larger Phoenix had a USL Pro team, Phoenix FC, which folded after 2 years. There's no shortage of venues in Tucson. With 56,000 seats, Arizona Stadium would be well-suited for MLS fixtures. The smaller Kino Veterans Memorial Stadium, which hosts FC Tucson, and the baseball-specific Hi Corbett Field are more suitable options for an NASL team. The former hosted a friendly between Canada and Denmark in January 2013.

New Orleans, Louisiana
Population: 1.2 million (metro area)
A city with soccer roots tracing back to the 19th century, New Orleans is currently home to NPSL (4th tier) side New Orleans Jesters, whose head coach, Kenny Farrell, voiced his support for an MLS team. In 2007, a group of investors conducted a study to see whether New Orleans had a potential market for an MLS franchise. Algiers and City Park were named as potential locations for a stadium. Indeed, only one existing venue would be suitable, and it just happens to have a bit of soccer history. Although now primarily an NFL stadium, the Mercedes-Benz Superdome once hosted a friendly between the New York Cosmos and Dallas Tornado, featuring Pele and Kyle Rote, Jr.

Detroit, Michigan
Population: 4.3 million (metro area)
Detroit has an interesting soccer history dating back to the late 60's, when the Detroit Cougars, a team featuring players from Glentoran FC of Northern Ireland, played two seasons in the United Soccer Association. Next came the Detroit Express, a team that played in the NASL and ASL in the 70's and 80's. Their inaugural season featured a European tour and the famous George Best and Bobby Moore as guest players. Soccer in Michigan is not a thing of the past, however. Detroit City FC, an NPSL club founded in 2012, already has ardent supporters in the thousands. Perhaps it's not so surprising, considering over 109,000 fans attended a friendly between Manchester United and Real Madrid at Michigan Stadium in Ann Arbor this August, the largest attendance at a soccer game in the U.S. since the 1984 Olympics. Detroit's Silverdome, which previously hosted the Detroit Express, is a potential venue that should not be overlooked. Although its future currently hangs in limbo and much of the stadium was auctioned off in March, the owners are "continuing to investigate all potential revenue options for the site.” They even applied for an MLS franchise in 2011. The arena hosted a friendly between AC Milan and Panathinaikos in August 2010. The challenge with an MLS team in Detroit? Attendance. The Seattle Sounders, who recorded a regular average attendance of over 40,000 in the past few years, are the exception rather than the rule in MLS. Michigan is one of the top 10 most populous U.S. states, but can an MLS team in Detroit draw regular attendance figures high enough to keep the team afloat and possibly boost the city's economy in the process?

Record crowd at Manchester United vs Real Madrid. Credit: Amir E

Minneapolis, Minnesota 
Population: 3.5 million (metro area)
Minneapolis is another city with NASL history dating back to the late 1970's and early 80's. The Minnesota Kicks, who played in Bloomington (suburban Minneapolis), regularly recorded attendance figures of over 20,000 and even 30,000. The Minnesota Strikers, a team from the Minneapolis-Saint Paul metro area, played in the NASL's last season in 1984 and had a short spell in the Indoor league. Today's NASL side, Minnesota United, is based in the same area. Founded in 2010, the team has supporters in the thousands and average attendance figures that jumped over four times in their first four years. When it comes to venues, the city has two arenas - Target Field and TCF Bank Stadium - that can potentially host an MLS team. The latter was part of the U.S. World Cup bid for 2018 and 2022. TCF Bank Stadium was also the site of the International Champions Cup friendly match between Manchester City and Olympiakos in August, drawing a crowd of over 34,000. There is a large ongoing campaign to bring an MLS team to Minnesota. MLS spokesman Dan Courtemanche said earlier this year that Minneapolis "can be an outstanding market - actually has been". League Commissioner Don Garber said in March that the city of Minneapolis "is on the short list" for MLS expansion in the Midwest.