As you know the original Marvel Phase One box set was delayed due to a silly lawsuit over the packaging. The set will now be released in a new case on April 2nd, 2013, and it's going to include some more cool stuff. Most notably a tease for Marvel Phase two.
Marvel Studios President Kevin Feige said in a recent interview with EW that it will include "footage, concept art, and a few new surprises" from Phase Two. As you already know by now Phase Two of Marvel's master cinematic plan includes Iron Man 3, Thor: The Dark World, Captain America: The Winter Soldier, Guardians of the Galaxy, Ant-Man and The Avengers 2. Who knows, maybe that Ant-Man test footage that was shown at comic-Con will be included as well.
The box set will also include deleted scenes that were not included in the previous Blu-ray releases, like an alternate ending to Thor, which you can watch over at EW. Feige went on to explain that, "We went back to the editor's assembly of each film and discovered some rare moments that dropped out of the films early on and had not been released on prior discs."
Marvel's Phase One box set includes ten discs of awesomeness featuring six movies that include Iron Man, The Incredible Hulk, Iron Man 2, Captain America: The First Avenger, Thor and The Avengers. The set will also include a 'working' Tesseract cube and S.H.I.E.L.D. files on most of the characters. Feige teased, "look for a nod to Iron Man 3 in the Tony Stark S.H.I.E.L.D. file in the box set." He went on to admit,
We spent an embarrassing amount of time compiling the ‘Shield Files’ included in the box set to be sure that each character's file was full of great, movie-accurate props and Easter eggs, which help tie all of the Phase one [Marvel Comics Universe] films together.
Sounds like something any hardcore Marvel fan will want to get their hands on!