I've been toying around with this idea for a while. There are so many directions that one could go, particularly with the economic issues of the nation and world, but also due to the fact that MLS is still in its growth phase and due to its relative cheap entry cost is being pursued by so many cities and ownership groups.
What we know is that by the 2014 Opener the league will have at least 18 teams, with at least 12 of those in soccer-specific-stadia in the 18-25,000 range. At least one stadium in the league will be of typical size for the major leagues around the world (EPL, Serie A, la Ligua, Bundesliga), with several of the teams having access to NFL stadiums for major matches.
We also know that the league will have English language TV deals with ESPN, and likely still with FSC. It will have a US-Spanish language deal as well. We know that in some markets MLS will be a third, or fourth, tier sport, and in others it will receive top level coverage on par with the legacy sports in the nation.
I really see a few different models that the league could become, each has their advantages, and at worst MLS will remain what it is.
Model One: MLS - the way it is currently, but bigger.
This assumes a continual gradual climb of the salary cap, moderate expansion of the league (up to 21 teams) and a continuation of general cost-containment with the single-entity structure. There will not be dynasties, and there would unlikely be any significant run in the CCL by any MLS club as the rosters struggle to be filled and fixture congestion remains an issues. Overall league quality of play would be at best 5th in the hemisphere (CONMEBOL+CONCACF)
Model Two: SPL - the same, but with "superclubs"
The main difference between the two models is that the single entity structure adapts a bit to allow significantly profitable or well run clubs to add a handful of players that are in their prime. This would allow cost-containment for the majority of the league, but would encourage a few clubs to try and take the risk of signing multiple "designated players" to hopefully compete at a higher level in CCL/SL and other non-league Cups. There would be a few of these superclubs something like Celtic and Rangers, but the majority of the league would be of relatively the same quality as currently (5th in hemisphere, but the superclubs could make runs to the Club World Cup).
Model Three: NASL - rapid expansion, end of single entity
This would cause a few clubs to fail, some clubs to relocate, and would end the relative parity and competitiveness of the league. If it worked the league might have 16 clubs with aging stars from Europe/Brazil/Argentina, the level of play would go up, but the league would be ravaged by stories of failures of franchises and ownerships going bankrupt. Overall league play would be raised, and the best teams in the league would go on runs, probably comparable to FMF in Mexico (tied for 3rd in hemisphere).
Model Four: Colombia - a league that specializes in selling talent up
Single entity structure remains, the designated player rule stays relatively unchanged, but the league focuses on remaining a stable investment for its ownership and players are transferred out to Europe to pay the bills, as most teams struggle to gain attendance and media ratings. Level of play might actually decrease, as college players choose the pine for lower leagues in Europe rather than playing here.
Model Five: Cash-Rich Single Entity - Remain Single Entity, but inflate the cap significantly and use luxury taxes to balance out the teams with Designated Players
This would actually combine the SPL and NASL models and likely involve greater expansion. While cost-containment would be an option there would also be superclubs that would use their wealth to not only build great teams, but also to help support the lower level clubs. This would be the most difficult situation to create legally, as it would take a rework of the CBA, league rules, and ownership, but would likely result in the most entertaining league play. There would be incentive if this were the style of league to grab expansion sites now while the fee is high, yet affordable to give clubs the monies to establish excellent academies and start the explosion of the cap over a five year stretch. If this worked the league could probably pass the FMF for overall quality.
Which do you feel is the most likely direction of the league, which direction would you like to see MLS go in, and what helps Sounders FC with their deep pockets the most?