Gerald Butler would make an excellent Hercules, but a Hercules movie, whether it be Marvel-related or Mythology-related, it is not the right time now. We've already had countless Hercules, Dracula and Tarzan movies. What purpose would another one be?
Furthermore, eventhough Hercules is part of The Avengers, it's just best that he be left out of the Avengers movies, because it would be ridiculous to include him. Look at him! He just doesn't fit, especially with all the countless Avengers characters that can be in an Avengers movie. He's just too dated.