I really think that NYCFC, Orlando FC and Miami will play a huge part in growing the MLS. Not just in size but popularity.
The player rumors surrounding these clubs and now coming to fruition with David Villa are enormous for the MLS. The fact that NYCFC is the 6TH? Largest club in the MLS based on twitter followers is a great example of this. They've grown by 10k since the announcement.
Youth soccer isn't a problem in the U.S. I have 5 fields within 10 miles of my house and they are packed with kids playing the game. The issue is once these kids get to middle school and highschool all they have to hold on to are those youth league experiences. They don't have any players to look up to and can't turn on the tv on saturday and sunday and watch their home team play. Why? Because the MLS more or less isn't televised. That's why most switch to football/basketball and so on. Look at youth league american football. Not nearly as popular yet the nfl is marketed much better and when kids grow up and get to middle school/highschool they want to play it.
The best thing NYCFC could do for the MLS and US for that matter is increase viewership by helping to establish a better tv contract (country wide) so that more games are televised.
There was a big thing last night about the portland game not being viewable to its own fans as it was on a paid channel and the free stream of the week. But the free stream didn't work for those who live in portland as it was a blackout so they literally couldn't watch the game without paying for the channel.
It's a big step in the right direction that's for sure.