2. Death
When Thanos popped up at the end of The Avengers movie, copies of The Infinity Gauntlet (Thanos' best book) began flying off the shelves as readers eagerly began finding out about this character. The Infinity Gauntlet's plot is pretty basic but also completely nuts: Thanos wants to impress Death in the Marvel Universe, she is a silent, pouty Goth chick/traditional skeleton in a robe and decides that wiping out half the sentient life in the universe, using the Infinity Gauntlet, is the way to do it. It seems like that book is a viable future storyline to explore in Avengers 3 Thanos is sitting out this Avengers movie so that the Avengers can fight Ultron, so he could be the big bad of the third outing. The Infinity Gems have already been referenced in the movies with Loki using what looked like the Mind Gem in the first Avengers movie and the Power Gem making an appearance in Thor: The Dark World. The Gauntlet itself can be seen briefly in the first Thor movie. If Marvel are headed toward the Infinity Gauntlet in Avengers 3, they're going to need someone to play Death could Kim Soo-Hyun be that actress? Marvel have a history of teasing appearances long before characters get the spotlight so including Death in Age of Ultron could be an early tease for the future Avengers: Infinity Gauntlet movie.