With Blade, X-Men, Hulk, Spidey, Daredevil, Punisher and Fantastic Four all getting the royal treatment, Marvel lawyers should get their act together and start bringing all the individual rights of Avengers related main characters into one company. There are three main problems with making an Avengers movie, all of which can be overcome with skill and perseverance, and money!
1-Characters like Captain America, Iron Man, Thor and Ant-Man have had their rights optioned off to different movie companies who do not care about the fans and would never be willing to allow their purchased character to be loaned to another company to make a team movie. These rights can and need to be purchased by Marvel to unify their heroes's rights under one roof!
2-Unlike X-Men whose characters and origins could be and were rewritten for the movies, The Avengers are bigger more established names with rich histories and the right casting will make or break the movie. All the super cool special effects or fancy costumes will mean nothing if the actors/actresses playing The Avengers are goofballs of the moment that the audience will laugh at. I believe all actors should be unknown and all should have serious training, preferably theatrical backgrounds. The Avengers onscreen need to be true to their comic book origins and legacies.
3-Stories! The Avengers gots some of the best ever. The right screenwriter(s) should be hired not to come up with their own take, but to translate and adapt great comic book classics to the big screen. My top candidate for big screen adaptation is Avengers Forever! That'd be just AWESOME!

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