In an interview with Desde Hollywood at the Los Angeles premiere for Iron Man 3, Joss Whedon has seemingly revealed that while he does indeed have plans for Thanos, he may in fact be saving him for what he describes as "The Grand Finale". It really depends on how you read into his comments, but it most definitely sounds as if the superhero team will have their hands full in 2015! Be sure to watch the video to also find out what Whedon thinks about the possibility of Robert Downey Jr. reprising his role as Tony Stark/Iron Man in the sequel. Check it out below.
"Thanos is more powerful. He's not someone you just trot out and punch him. Like you did in the comics, you want him to be threading through the universe and to save the big finale for the big finale. He's definitely a part of what I've got going on. The thing about The Avengers is, that they are very powerful, but they aren't very stable. There'll definitely be some people who are gonna shake them up in the next instalment!"