I was not a soccer fan growing up, but have over the last three World Cup come to appreciate the competition. This World Cup has been very enjoyable thus far. My thoughts:
1. A first-rate (read: English Premier League, La Liga, Serie A or Bundesliga) soccer league would excel in the United States. Don't let anyone tell you otherwise. Americans embrace the highest-quality version of just about anything. Relegation is something they would buy into whole-heartedly.
2. Epstein offers his changes for the soccer here; this isn't the first time, nor the tenth, nor the last time you'll see articles in this vain. You won't see changes in soccer until you see these sentiments in the places where soccer matters, just like you won't see a growing feeling of salary inequity from Argentinians affect large-scale labor change in Major League Baseball. As best as I can tell, the calls for modifying soccer are uniquely American. (Though please comment about other cultures that want to change the game, I'd love to read it.)
3. Given the success of Twenty20, I wonder if the best way of moving forward with modifying soccer isn't to create a new sub-sport in and of itself. There have been other instances of such attempts-- what comes to my mind is Arena football and the XFL, though I'm sure there are others-- but to me, those seemed to be modifications that tried to keep the part of a sport that people liked (fast/big plays) while adding in gimmicks to attempt to differentiate it (indoors, cameras everywhere, no fair catches, no punting, etc.). Twenty20 kept the sport the same but got rid of what people didn't like (the length of a cricket match). Maybe that's splitting hairs, but to me it's an important difference.
4. Given #3, perhaps a modified game that compliments soccer would be successful-- or rather, if action had to be taken, this would have the best chance of succeeding. Like above, keep the game and get rid of what people here don't seem to enjoy. Per Epstein, find a way for more scoring, less importance for referees but keep the games commercial-free by half.
Ultimately, I think compliment instead of substitute is the way to go.