Thanos will be a main villain in the avengers 2 as after the first bit of credits of the new Avengers film it pans across a scene of space and then across to the side of someones head (who looks like hellboy or red skull etc.) but he then turns his head revealing that its thanos, indicating that he will appear as a main role in the next movie.
However not many people have noticed this as its a few minutes into the credits and most people wouldve left the cinema before then.
To be honest my first thoughts were that it was hell boy but after having a think about it it deffinately looked more like thanos. but then again it could be red skull since he was in captain america, but the guy deffinately looked more like thanos which will be good because theyre bringing new enimies into the film series because in The Avengers the main enemy is (spoiler alert) loki which was a bit of a letdown as hes in a lot of avenger films.
Ps. i have actually seen the film so im not actually making all this up
hope this helps.