When you have a about a superhero team comprised of Iron Man, Captain America, and Thor (and others TBA), then you're going to need a serious antagonist for Joss Whedon's The Avengers.  So how's a race of an extraterrestrial shape-shifters bent on conquering our planet work for you?  ComicBookMovie.com has "several insider reports" that say the Skrulls will be setting up an infiltration in the first film with a planned sequel heralding a full-on invasion.  It was previously rumored that the plot would have Thor's villain, Loki, casting a spell on Hulk so that he would be facing off against the Avengers.  If this new rumor is to be believed, the Skrulls would brainwash Hulk for the same purpose.  Additionally, the alien race, "The Kree" will be allied with Earth against the Skrulls.

Hit the jump to learn more about the Skulls and the Kree.  The Avengers is slated to hit theaters on May 4, 2012.

Here's the official Marvel bio for the Skrulls:

'The Skrulls are an extraterrestrial humanoid race who have created a vast interstellar empire, the oldest known empire still in existence, within the Andromeda Galaxy.. A Skrull can mentally cause the unstable molecules that comprise his or her body to become pliant, enabling him or her to assume other forms through muscular expansion and contraction. Once a new shape has been assumed (human, animal, alien), it takes a conscious act of will to assume another form or revert to natural.. Although the Skrulls became aware of Earth (due to its nearness to a natural warp-space access point) eons ago, it was not until recent times that they considered Earth’s threat. Their first regular surveillances of Earth began in the 1930s. To date most of the Skrulls’ excursions to Earth have been extremely small-scale, with no more than a handful of soldier-infiltrators being dispatched at any given time. It is not yet known how the death of the current ruling family will affect official policy toward Earth or the government of the empire in general. '

And here's the Marvel breakdown for the Kree:

The Kree is an extraterrestrial humanoid race who have created a vast empire in the Greater Magellanic Cloud and have, on certain occasions, trafficked with the beings of Earth. The Kree race began on the planet Hala in the Pama system long before the first mammals appeared on Earth. The original Kree had blue-colored skin (later generations interbred with genetically-compatible aliens to produce the pink-skinned second race). Outwardly humanoid to a large degree, Kree bodies are adapted to environmental characteristics on Hala that are un-Earth-like: notably, higher gravity and higher nitrogen content in the atmosphere. Although the Kree cannot breathe Earth’s atmosphere without special apparatus, their denser bodies afford them about twice the average human being’s strength and endurance. Despite their physical superiority and relatively advanced technology, the Kree race has reached the pinnacle of their evolutionary development.