Some of the most difficult things to do on Earth: Climb Mount Everest. Make a Fantastic Four film that fans don't hate. Honestly, I think people have a better chance of climbing Mount Everest without any gear than they have of making a fan-pleasing Fantastic Four movie. That is of course not fault of the actors, especially Kate Mara who played a very interesting younger take on Sue Storm compared to Jessica Alba.
The Invisible Woman is an interesting character because you have to cast someone who believably wouldn't use their powers for evil. If you looked at me and found out I could turn invisible, you'd be like, "that guy is going to do pervy things, isn't he?" But thankfully that is not the case, but I would totally use my powers for pure evil.