What's goin on Internet? I'd like to start off by saying that I haven't read a comic book (outside of Watchmen) in probably 15-or-so years. Everything I propose here is based off of what I've seen in all of the Marvel films.
I've been a pretty big fan of Marvel's current library of films, specifically the massive universe that they're building in anticipation of The Avengers. In order of timeline: Iron Man, Iron Man 2, The Incredible Hulk, Thor, Captain America. In order of my fondness of them: Captain America, Iron Man, Thor, The Incredible Hulk, and Iron Man 2. Over the course of the varying films, SHIELD and the concept of The Avengers has been slowly becoming more of a focus for the franchise. Iron Man 2 really got the ball rolling, with Thor and especially Cap bringing the concept of The Avengers fully to life. I, as have millions of fans, have been trying to piece together what could possibly bring all of these great heroes together...basically any plot threads that might pop up in the film. I'm going to share with you all my thoughts on what we just might see this May...
Captain America - A lot of the groundwork for what I believe will come to pass what laid in Cap, last year. We all know that Red Skull was looking for, and found the Cosmic Cube; presumably Odin gave it to the people of Tonsberg, Norway, as a gift after the assault of the Frost Giants way back in the day (events that played out during the beginning of Thor). As we all know, Red Skull and Dr. Arnim Zola were using the Cube's insane amount of energy output to power Zola's weapons. At the time, and I still stand by this, I made the observation that Zola's weaponry sounded similarly to and had the same bluish hue as Tony Stark's arc reactor technology and his repulsors. This idea is corroborated with the fact that after Cap crashes Red Skull's mega plane, Howard Stark recovers the Cosmic Cube from the floor of the ocean. He was already messing with the tech from Zola's weaponry earlier in the film. Its safe to assume that he himself reverse-engineered the tech from the Cube to power his arc reactor. Its also quite possible that the Cube's energy output helped Howard to discover Vibranium, which Tony had to re-discover in the events of Iron Man 2.
Thor - Most of the events of Thor don't have much foreseeable impact upon The Avengers...its safe to assume that part of the film's plot will revolve around getting Thor back to Earth and rebuilding the Rainbow Bridge/bifrost. The after-credits-scene clearly sets up Loki as the villain of The Avengers (which we've known for quite some time) and first introduced us to the Cosmic Cube. Until Captain America came along, we had no real idea of what the Cube was capable of (within the context of the films). I fully believe that the Cube is capable of creating bifrost, allowing one to travel to the far reaches of Yggdrasil. This would mean that Red Skull is chillin' somewhere in one of the many realms of the World's Tree (as per the ending of Captain America). At the climax of Thor, Loki lets go from Odin and Thor to fall into the vast reaches of Yggdrasil. It must be pretty rough out there, because when he shows up at the end-credits-scene he's not looking too great. The revelation that both Red Skull and Loki are chillaxin somewhere in the cosmos leads me to believe that they've run into each other at some point and are possibly teaming up.
The Avengers - We don't know a ton about the plot of the film...just that Loki is messing up Earth with an army of some sort. Kevin Feige has said that neither Red Skull nor the Skrulls will be in the film. I find that hard to believe. I have a very strong feeling that Hyrda will show up somehow. In the Super Bowl trailer, you can see flying ships of some sort; they're either alien in nature (probably from somewhere in Yggdrasil) or they're commandeered by modern day Hydra. Hydra would make an excellent fit as the army commanded by Loki; Joss Whedon has said that Steve Rogers will be more or less the main character of the film, considering that he's new to our world and we'll be seeing it through his eyes. Making Hydra a force in the film would give Steve motivation; having just fought them in the 1940s he would be able to make some sort of connection to this new modern world. All of these "ships" have a bluish tint to them, indicative of Tony's arc reactor tech and Arnim Zola's weaponry of the past. It would make sense if some of that technology survived through to today, with remnants of Hydra still around. As for the green/brown aliens in the trailer..no idea on that one. I'm not versed enough in the comic lore to make a guess. If that's Loki's army, then either they are piloting those ships, or Hydra is involved as well. If Hydra is involved, I would be hard-pressed to believe that Red Skull is not in The Avengers If he and Loki met somewhere in the cosmos, they would most likely have formed a pact to take over Earth, leading to Loki controlling Hyrda forces.
This is all merely conjecture; there's a strong chance that all of this will turn out a different way and I'll look like a fool. Whatevsies. Let me know what you think in the comments!