At the moment you have all kinds of rules in place like the salary cap and restrictions on how many international players you can have etc. Whilst this is good for American football in general, so the American players can develop their game and play in the MLS, it doesn't make the MLS a better league.
The DP rule is a good rule as it allows players like Henry, Beckham and Marquez to come and play in the MLS when they're approaching the end of their careers. But somehow, the MLS needs to overcome that and get the stars coming over in their prime. The US lifestyle will appeal to players from over here and in Europe. The big city life in the US is very desirable. But unless serious money is thrown at it, like it was when Pele and co played for the Cosmos back in the 70s. That's when 'soccer' was huge in the states and appealed.
They're trying to change the rules here in England so that you have to have home grown players playing for your clubs, so that the national team can improve. Up until recently, teams like Arsenal, Man Utd, Liverpool and Chelsea here would often field teams with one, maybe two Englishman in. Because there's a lot of talented players abroad, and they make our league a lot better and appealing worldwide. But with the talent comes a price. Like our top earner in the league earns something like £250k a week. Unless the MLS became big like that, I can't see it being big in the US or worldwide. I really want it to, as I want the game to progress in America and for the stars to also play out there for a while. Unless the best players come over, I don't see it being amongst the Premier League or La Liga.
Also on a sidenote - You often see American 'soccer' fans follow a team over here in England or perhaps in another European league, but will also watch the MLS as it's on their tvs over there and their nearest chance to watch a game of football. But they often follow the European team as their no.1 team and the MLS team as their second team. If the MLS wanted to be a major competitor they'd have to somehow become the no.1 team for their local fans and attract international fans too.
People here in England follow the MLS to see how their old players are getting on. Like Thierry Henry is a hero of mine. Being an Arsenal fan, he's like a God. He's our record goalscorer in our history and since he's been in New York, i've taken an interest in the Red Bulls. I also follow the Fire as they're from Chicago, and i'm a Bears fan so it just makes sense to stick with that city. But when the likes of Blanco and Ljungberg play for them, it's attractive (especially Ljungberg with the Arsenal link again). Just the legacy these great players leave in Europe, can be a big attraction to us European fans when they move across the pond.