If they're going to do full opening credits... how about a refresher of sorts of the time between the end of the first movie and the beginning of the second?
They could have a mix of live action and still photos (or artwork like the Spider-man movies) that detail what's happened to the team since defeating Dr. Doom. Highlight some crucial battles, some accolades from the media and fans... establishing them as the premiere heroes in the Marvel movie universe.
Here's where I think you can show what happened to Doom without really having to spend time developing his restoration. Press clippings and images in a moving collage would be most effective.