Another advanced alien race, the Skrulls have spent most of their history at war with the Kree. While the Kree are genetically stagnant, but technologically advanced, the Skrulls posses shape-shifting abilities, and so are constantly changing and evolving from one appearance to the next. The Skrulls have invaded the Earth several times, often by posing as it's greatest heroes and protectors. As a result, they've come up against almost every group of heroes in the Marvel Universe, including the Fantastic Four. The Skrulls also have a mutant population similar to humanity (in both cases, this was the result of genetic tampering by the alien Celestials). This means that the Skrulls have a level of unpredictability that stops them from becoming boring. Another group within the species are the Super Skrulls, genetically altered warriors that posses both the species' original shape-shifting abilities, and the combined powers of the Fantastic Four. Ok, so they sort of touched on this in Rise of the Silver Surfer by giving Johnny Storm all of the team's powers, but didn't really use it to the full extent.