4. Tease a Villain More Badass Than the One Currently in the Movie
Perpetrator: The Avengers, Sherlock Holmes I'd be lying if I told you that before I actually saw Thanos in the mid-credits scene of The Avengers, that I expected him the whole time to be the man behind it all. It caught me by surprise and it was awesome. But they do allude to him quite often, as they do with Moriarty in the first Sherlock Holmes movie. And as much as I love both those films, now, whenever I watch them, I just want the other villain to show up and cause all the big, bad destruction that would be unparallelled by the current antagonists. Which isn't a knock (I know I say that quite often) to the performances of Tom Hiddleston and Mark Strong, because I greatly enjoyed both of them. I just know the destruction characters like Moriarty and Thanos would cause if given the chance. And knowing that they're there the entire film, hiding in the shadows and observing, it makes it all the more plausible (in "film reality") that they could appear at any time and up the ante. Take a note from Batman Begins: introduce the next big bad at the very end in a clever way that doesn't damage the rewatchability of the first movie.