There are three things that I want to be clear on before jumping fully into the conspiracy theories. If you’ve read the comics (specifically The Infinity Gauntlet), then this may be unnecessary for you. If you haven’t – and SHAME ON YOU!!! – you may find this section useful.
Item #1: Loki is the God of Lies and Mischief. I don’t think they ever explicitely state that in the film or how obvious it is to non-comic book readers, but that is very important. Also, as an aside, his goal is to have the throne of Asgard. That should be obvious to anyone whose seen the movies, but it is very important and is worth repeating.
Item #2: That big purple guy from the end of The Avengers is the best thing that has happened to these movies: That is Thanos. Those few seconds with Thanos are probably the most important seconds in the entire movie (because of Thanos and one other thing in that frame… but that’s farther along. In the comics, Thanos is from the moon Titan and is very power-hungry. The second most important thing to know about him – and his reason for being power-hungry – is that he is trying to win the love of Lady Death. (May I declare that the last line in this clip is the best line in the entire movie.
Item #3: The Infinity Gauntlet is a very powerful weapon that basically lets the wearer have power over time, space, reality, minds, souls, and power. It is made up of the six Infinity Gems, each of which are related to one of the aforementioned fields. One of the more notable Infinity Gems is the Mind Gem (the blue one). Oh yeah, in the comics, Thanos created the Infinity Gauntlet.
We’ve seen Loki in two movies and Thanos in the end credits of one (and he will probably be in Guardians of the Galaxy in 2014 as well). However, less obviously, we’ve also seen the Infinity Gauntlet as well.
(From the scene where the Jotuns are running from the Destroyer after trying to steal the Casket of Ancient Winters in Thor)
These three pieces in Joss Whedon/Kevin Fiege’s massive chess game are interlocking. With the Infinity Gauntlet, Thanos might be able to win over Death’s love. However, the Infinity Gauntlet is in Odin’s treasure room, which Loki might have access to if he were king of Asgard. But Loki is not king of Asgard. A bargain between Thanos and Loki for an exchange of the throne of Asgard for the Infinity Gauntlet would seem plausible, no?
Note: Read The Infinity Gauntlet. I wasn’t sure about it based on the general premise of “all the heroes die” reminded me of the Heroes Reborn storyline that I encountered while reading the Clone Saga. I was VERY surprised by how good it was. Still need to read Infinity War and Infinity Crusade, however.