MLS will expand into whatever market is willing to pay the expansion fee. If there are 20 groups out there willing to invest $75 Million, they will add the bulk of those investment groups. They are not in a position to turn down capital injections. If Carolina, Las Vegas, Detroit, St. Louis, Carolina all pony up the franchise fee, they will all join (and relatively quickly).
Point of order, the most recent expansion fee was rumored to be in the $200mm range.