In May, MLS announced a second franchise in New York. That elicited this article on MLS expansion. It’s likely that MLS will have 24 teams within the next 10 or 15 years.
Since then, there has been a lot of water under the bridge.
MLS VP Dan Courtemanche said future expansion will be addressed in the near future, via Joe Prince-Wright of NBC's Pro Soccer Talk. It appears we are inching ever closer to a David Beckham-owned, South Florida-based franchise.
There have been constant rumors (although they have usually been adamantly denied) that Chivas USA or another MLS team could pull up stakes for greener pastures, via Ives Galarcep of Soccer by Ives.
Following a user request, I have added additional markets, plus additional information on market size and outdoor minor league history, and ranked them this time. Though we may eventually get second franchises in some markets, each is in a market without MLS.