Once again, MLS wants a soccer specific stadium. They will not allow them to share the stadium with another sport. Even then it's going to fit around 20k with a chance of expanding to maybe 30k years down the road. I'm talking many many years down the road.
It will not happen. Sorry to burst your bubble.
Sorry, you are talking out of your ***. As someone else posted, Beckham has stated that he is open to working with UM. They also want it big enough for major concerts. As you state, the MLS policy is that the stadium be soccer
-specific, not soccer-
only. So, it must be designed for soccer, but can also be designed for other things. Case in point-- the Seattle stadium that was designed for the Sounders and the Seahwaks. And oh, by the way, the Sounders have the highest attendance in the MLS with a great atmosphere at their games, and the Seahawks have the best home field advantage in the NFL. It is way easier to make a dual-purpose for soccer and football than baseball and football. The dimensions are pretty close for soccer. Baseball is different-- that's why Sun Life's set up sucks so much for football.
There are definitely talks going on between UM and Beckham. Blake James and Shalala were at the MLS announcement party. Both Beckham and Blake James have stated they are open to working with each other for a stadium. Someone posted the link for the Beckham story. Here is the link for where Blake James said it:
http://blogs.palmbeachpost.com/caneswatch/2013/11/29/miami-ad-were-interested-in-david-beckham-soccer-stadium/
Finally, I would just say that I seriously doubt Beckham is going to be able to get prime real estate for a venue that only has MLS games. They need UM in on this.