I've decided to take another crack at this contest, so let's see how this goes with one of the biggest superhero movies ever, The Avengers.
Now, we all know that The Avengers ends with Tony Stark nearly dying, but the Hulk just narrowly saves him. They then capture Loki, and take him straight to Asgardian jail.
Now, my idea is.. What if a certain wall-crawling, friendly neighborhood Spider just happened to be in the right neighborhood at the right time?
That's right, what if Spider-Man was the one that saved Tony? What if none of the Avengers saw Tony falling, or, what if Hulk missed? Spider-Man would have saved Iron-Man, and made allies in the Avengers that day. That would certainly set up for Civil War. They head up to see if they could find Loki, after a talk with Spider-Man (*Cough* Avengers Initiative. *Cough*) and find that Loki has disappeared. Thor pledges to find him, which somewhat sets up for Dark World/Ragnarok, and everyone leaves happy.