^Ryan's got the best points.

Disney doesn't need Vader to sell this movie. You know what's a more iconic image than Vader? The Star Wars logo. Boom. That's all they need.

What Disney's primary goal should be is to make sure that this film is good. If it's bad, and they keep making bad Star Wars movies, the brand will die. People will stop seeing the movies. The only way to keep high revenue with their plan of Marvel-esque yearly releases is to keep the quality high. They almost certainly understand this.