Why especially if they include Thanos? Not many people (enough to make a dent anyway) are clamoring for (or are aware of) Thanos.
Well I think Marvel has pulled off making lessor known characters popular. Thanos is a total unknown to 99 percent of the world save for Marvel comic fans. Despite that he is a good character and can be an awesome villain. He's complex and has a rich back story. Written and presented correctly he can shine in a film. Marvel will introduce him and pull it off. He's the just the right villain needed for an Avengers film. The smaller Earth based villains are suited better for the solo films. I always felt that Thanos would been a good villain in a stand alone SS film but it'll never happen cause Fox has the SS. I like that Marvel is trying to introduce him into the movie verse, especially with the news that A Guardians of the Galaxy film is in the early stages of production.