Readers of the classic Marvel comic storyline "The Infinity Gauntlet", an adaptation of which is most likely the ultimatum of the current storyline taking place in the Marvel Cinematic Universe, might remember the important role that Mephisto played. In the Infinity Gauntlet storyline, Mephisto stays at Thanos's side and tries manipulating him, feigning subordinance so that, in the end, he could claim the infinity gems for himself and rule existence.
Now The Avengers came out a few months after Ghost Rider 2, so Mephisto was still in the hands of Sony. Anyone with half a brain knew that a barely marketed Ghost Rider movie released in January was bound to flop. Thus, for both legal boundaries and the sake of slowly building the universe, Marvel couldn't use Mephisto in Avengers. However, they began to plant the seeds... by introducing the character known as "The Other".
In The Avengers, The Other gave Thanos the scepter and the Chitauri army. I always assumed he was the leader of the Chitauri or something until I saw this pic of him and he looks almost human, except he's wearing a mask that blends in well with his face and makes it look like its part of his facial structure. What is there to hide, exactly?
The Other was played by Alexis Denisof, who will be reprising the role in Guardians of the Galaxy. Obviously, this character is important enough to be portrayed by the same actor across multiple villains, and isn't simply a minor plot point to be thrown away. Denishof himself said in an interview, "A seed is certainly planted there for all to see. Of course, I can't say much more than that. However there is a lot of talking going on right now about where it's all gonna go."
While The Other, on the outside, seems to simply be a servant of Thanos, Mephisto has no problem appearing to others in a way that they'd like to see. Even as a Satanic type demon, Mephisto couldn't possibly have the resources to take over the universe himself, so he gets his way by manipulating others. Thanos, in theory, would be INCREDIBLY easy to manipulate, given his love of Death. Help him acquire the means to Death's affection and he'd, at least unknowingly, bow to your every whim.
A story arc of this magnitude can very well expand beyond the three Avengers movies. While some like to compare Thanos's role in the MCU to Emperor Palpatine, if my prediction were to come true, Thanos would become the freakin' Count Dooku of the universe, while The Other's role is akin to that of Darth Sidious in the prequels, encouraging others from the security of a dark hood until he is in a position of complete power. Except encouraging more threatening things than raising taxes on space tea.
Theoretically, Mephisto take control of the Infinity Gauntlet after Thanos loses it. Post Avengers 3 movies could deal with the heroes literally needing to take down the mother****ing empire of Satan, allowing supernatural characters like Doctor Strange, Ghost Rider, and Blade to flourish. If it doesn't go that angle, but The Other is still revealed to be Mephisto and dies in Avengers 3, it would still be a cool plot twist.