In a surprising reveal, Joss Whedon tells Desde Hollywood that despite the hint of him during the end credits of "The Avengers," don't expect Thanos to serve as the villain for at least the first sequel anyway. Whedon says:
"Thanos is more powerful. He is so powerful; he is not someone you can just trot out and punch him. Like he did in the comics, you want him threading through the universe and to save the big finale for the big finale.
He is definitely a part of what I have got going on. The thing about The Avengers is that they are very powerful but not very stable. So, there will definitely be some people that will shake them up in the next installment.
This essentially indicates Thanos might not arrive on the scene until Marvel's Phase Three plans - "The Avengers 3" in 2018 then?