The Next 4

Last night MLS commissioner Don Garber announced that MLS will expand to 24 teams by 2020.
So that begs the question: which 4 cities will it be? Let me first say, I'd love to see Canada (the likelihood is that it would be Ottawa) get another team but I highly doubt that will happen. I also think San Diego is a great market for MLS, but I'm confidant that Chivas USA will end up there. Hopefully with new ownership and a complete re-brand. So those 2 market are off the list.
With that said: Here is the 4 markets I think MLS should expand to.

Orlando: Orlando City FC have been a success in the USL since their entry in 2010. The club has loyal supporters, they've been good on the field and, the ownership has been trying to get a 20,000 seat soccer specific stadium build in an attempt to get to MLS. MLS also folded their Florida state teams in 2001 and have been wanting back in the state and this market is the closest to being ready for MLS in Florida.

Atlanta: Since 1995 the Silverbacks have been in Atlanta. So right off the bat, like Portland and Seattle, there is a proven history of being able to sustain a franchise in the market. In 2006 the Silverbacks built their own stadium (currently 7,500 seating)  that has been designed to expand seating up to 15,000. The complex includes a several international sized practise fields, a club house, pool and, fitness center. So they are all ready to go for MLS they just need to expand the stadium. Another side point is that Atlanta is the biggest media market without an MLS team.

Minneapolis-St. Paul: It's been a little bit of a weird history in Minneapolis for soccer. In 1990 the Minnesota Thunder were founded and played in the USL-1 (which was teir 2 at the time) when the USL was shutting down and making way for the new NASL the Minnesota Thunder folded. Luckily, NSC Minnesota (a completely different franchise) came to be. in 2011, the team was going to be operated by the NASL (as opposed to the National Sports Center). 2012 saw the team name change to the Minnesota Stars and they became NASL champions. Most recently the team has been sold to private ownership and has been renamed Minnesota United FC. There is no soccer specific stadium large enough in the area at this time, which might be a problem for MLS but it's clear that since 1990 tier 2 soccer has been in the area and I'm sure that MLS would do well in the twin cities.

Carolina: Another city that has proved they like soccer. The Railhawks have been playing since 2007 and have developed rivalries with the Rochester Rhinos, Atlanta Silverbacks,Charleston Battery and, Puerto Rico Islanders. Their home field, WakeMed Soccer Park, is a 10,000 seat stadium that could easily be redeveloped into a larger stadium for MLS. All the ingredients are there.

That said, David Beckham has been rumored to  be taking meetings in Miami about the possibility of getting an expansion club in Miami (Beckham was promised that he could have an MLS club when he retired as part of his MLS contract) so I wouldn't be surprised to see a club in that market either.

Possible MLS Locations