Looking at the map of how the teams are laid out across the States, two key areas jump out the Southern States east of Texas, and the, er, top middle States (Montana, the Dakotas, Minnesota, etc). Is there any untapped potential there?
There could be, but the biggest problem there is that this part of the country doesn't have that many big cities and the population tends to be very spread out. There are NBA teams in Salt Lake City and Oklahoma City, neither of which has another major sports franchise. But looking down the list of major media markets, these states are not well-represented. None of the biggest cities in Montana, the Dakotas, Idaho or Wyoming are in the top 100 media markets.
College football is big in those parts of the country (hell, it's big in every part of the country) but each team only has about a half-dozen home games a year, so traveling to the games is a mini-vacation for the fans. Not to mention that those schools have alumni going back decades who have strong ties to their alma maters. One can't really expect much of that kind of dedication to travel during a 30+ game soccer season for a team that didn't exist the previous year.
Weather also could be a factor there. The Minnesota NFL team plays indoors because winter weather there is often severe. Their baseball season overlaps closely with MLS's season and the early and late games of the season are often played in cold weather. But the baseball team has been there for 50 years and has an established fan base. Looking at the map, the Twin Cities (Minneapolis & St. Paul) are the only major media market in that portion of the country that doesn't have an MLS team already. I don't know how much demand there would be.
In the South, the most obvious cities that are capable of supporting a team are Atlanta, Tampa, and Miami. Atlanta is a lame sports town by any measure and I doubt there's much demand there. (How Atlanta won the 1996 Olympics still baffles me. Ted Turner and Coca-Cola must have flat-out bribed people.) Miami used to have a team, but it didn't do well there. Miami also has a middling sports tradition, with only the NFL's Dolphins having a dedicated fan base. (Again, that team has been there over 40 years.) Twenty years ago Miami didn't even have baseball, basketball, or hockey(!) teams and none of those teams has had solid fan support so far.
The one other potential southern market might be Charlotte. It is a big media market and they do have a tradition of solid college soccer programs there. I have no idea how much attention it could get there though. It's the epicenter of NASCAR.
Alternatively, while there's no promotion or relegation to or from the MLS, could some USL teams be 'upgraded' to become MLS franchises? They're established teams so one would imagine the core support is there already, would developing these sides to be future MLS members be a less risky proposition than planting a brand new team in untested territory?
I suppose it could be done, but the USL team support bases are very small compared to the MLS teams. If you upgraded one of them to MLS, it would still be the equivalent of an expansion team playing against the established teams for the at least for the first couple years. And the way player contracts are done in MLS, most of the roster would be overturned anyway. More likely they would just establish a new team in the city and the USL team would move or disappear. Kind of counterintuitive, but we have no tradition of any kind of promotion here.
