-The aliens are called The Redacted and are a new version of The Skrulls. Fox still own Skrulls so Marvel have changed them enough to not get sued. They're multicoloured and are shapeshifters.
-Thanos is in the movie. There were rumors some months ago that Loki would do business with Thanos and they're true. Thanos is tricked by Loki in the movie. Basically, Loki has been collecting Infinity Gems and the Infinity Gauntlet, but tricks Thanos in to thinking that the Infinity Gauntlet is the most powerful. In return, Thanos gives him The Redacted.
-The Leviathan is an alien serpent given to Loki by Thanos as a part of the deal.
-Red Skull is in the movie. Loki meets him on his travels. They split the 6 Infinity Gems between them. Loki takes the Mind, Reality and Power gems and Red Skull takes the Soul, Time and Space gems.
-A lot of The Avengers was shot in secret. These are where Red Skull's scenes were filmed.
-The Redacted are Loki's army. HYDRA are Red Skull's army.
-The Redacted discover that Loki tricked their master, and betray him. Hence Loki seemingly being on The Avengers side in some set photos.
-The French trailer descriptions floating around the web are TRUE
-Lastly, in the email there was a detail being withheld. He said that if Marvel ever found out he leaked the following, he would never be hired again. It's bigger than everything he said before. He didn't tell me, although he gave me this hint:
"What happens when you piss off one of the most powerful beings in the universe?"