San Jose in 2008 is official, making the West Coast just as hefty as the East with a grand total of 3 MLS sides. Here's who I would pick next:
The Pittsburgh Riverhounds are the next expansion side to join USL Div. 2, where the Harrisburg City Islanders have already staked a claim. I am positive there would be financial backing, and if the team were sound enough, I think a significant contingent of southern New Jersey fans and northern Maryland fans (and think about Delaware!) would siphon off the NYRB/DCU bandwagons and head Philly-ward for some MLS action. But here's why PA is on the top of this list: Pennsylvania is a phenomenal soccer hotbed with more than its fair share of youth (read: ODP, Super Y-League) teams in place. With perennial powers NYRB and DCU as rivals right off the bat, I think Philadelphia would blossom not unlike a certain Canadian side that has skyrocketed this season.
Footy fans in the Pacific Northwest are positively rabid for an MLS expansion to come their way. Fed up with the hype-fest that is Los Angeles, they've lined up not one but two potential corporate backers, the latter of which displaying typical American panache by using the moniker 'Atlético Seattle.' Furthermore, the Pacific Northwest is a proven soccer breeding ground as the Timbers, Sounders, and Whitecaps all play USL Div. 1 soccer in Oregon, Washington, and Vancouver respectively. The only reason I don't place Portland, Oregon or Seattle, Washington at the top of this list is because I'm not sure how enthusiastic MLS will be about a fourth team on the West Coast on top of San Jose, Chivas, and LAG.
Well, if we're going to talk about reviving the Earthquakes, we have to talk about reviving the Miami Fusion. There is a fantastic Latino market that yearns to be catered to again, and with the disgustingly impressive amount of disposable income floating around down there, I have to think that Miami is the perfect tropical location for a soccer team to grow. Rather as a side note, Miami F.C. plays USL Div. 1 soccer down there already, though they don't enjoy stellar game attendance.
4) New York
The soccer gluttons in the Big Apple are already clamoring for another team besides the New York Red Bulls, and I happen to know that Red Bull isn't filling the stands at Giants Stadium. NYRB hasn't monopolized the New Jersey soccer market, either: St. Benedict's Prep of Newark, NJ has established itself as the premier high-school soccer development academy outside of Bradenton, and NRYB aren't snapping up all of its graduates. Furthermore, the immigrant and low-income populaces of Brooklyn, the Bronx, and Queens in NYC have foundered a fantastic place for a star to be born, replete with concrete futsal pitches and infinite gringo ankles to be broken. The only reason I don't put NYC higher is because I'm not sure how gung-ho MLS will be about bringing up another side catering to the NYC market-- but hey, it's working for Los Angeles, right?
5) Las Vegas
I'm positive there would be significant financial backing for a potential MLS side in Sin City, but I'm just not sure how much fan support LV could generate. That, and all the potential stadium plans I've heard about, no matter how often they endorse the term "family friendly," involve a casino of some sort. A few intrepid souls have already laid the groundwork for an MLS side to arrive, but really the only thing attractive about an MLS team in Las Vegas, Nevada is Las Vegas, Nevada.
6) Somewhere in the Carolinas
The only reason I say this is because both North and South Carolina boast Div. 1 sides (the RailHawks and Charleston Battery, respectively) which enjoy outstanding fan support. From what I've heard, though, there isn't any corporate backing and the amount of support for both teams seems to have hit a glass ceiling. On the other hand, NC, much like PA, has a healthy amount of youth sides fostering support for the USL teams. I'm on the fence with this one, I guess.
I don't think this is going to happen, but I'll put it on this list if it will help Peter Wilt calm down a bit.
I found this on duNord awhile ago: a map of all the MLS and USL Div. 1 teams in the US and Canada, so you can peruse the concentration of professional teams for yourself.