If you stayed for the credits at The Avengers you must have seen that “clue” offering a look into the sequel, featuring some non-humanoid monster which I thought looked an awful lot like Hellboy.
Of course, it isn’t Hellboy. What would a paranormal investigator from earth be doing floating around on a rocky planet in deep space?
Dark Horse Comics has no relation to Marvel and there was never a “Hulk vs. Hellboy” or “Black Widow vs. Liz Sherman.” Obviously Ron Perlman and Selma Blair would decimate Mark Ruffalo and Scarlett Johansson, but that’s beside the point.
The creature in that sequence is actually the titan deviant Thanos. A key storyline in the Marvel universe occurs when Thanos takes control of the universe with the Cosmic Cube aka Tesseract and attracts the attention of Mistress Death.
He is eventually defeated by The Avengers and a few others. Thanos is also associated with another major universe-mastery item, the Infinity Gauntlet, which could be a key element in the next movie.
[Here’s one more photo from the sequence, it’s very similar but worse quality and he’s grinning more.]