3. Civil War
Civil War would be a great way to merge the three franchises in question. The premise of Civil War involves the introduction of a Superhuman Registration Act in the United States of America. This causes conflict, as two groups - one pro-registration and one anti-registration - emerge, prompting countless superheroes to come out of the woodwork. It would be a simple way of making the likes of the X-Men, the Fantastic Four, Spider-Man and the heroes of the Marvel Cinematic Universe make themselves known to one another and, depending on which heroes choose to register and which opt not to, it would create a mixture of the characters on either side. For example, in the comic book arc, Tony Stark leads the pro-registrations group and Captain America leads the anti-registration group. Heroes like Black Widow, She-Hulk, Wasp, Mister Fantastic and Henry Pym sided with Tony Stark, while heroes like Cyclops, Falcon, Luke Cage, Spider-Man and Nick Fury chose to side with Captain America. The two sides end up battling in an epic war against each other and Captain America and Iron Man have a particularly awesome fight. It's another ideal opportunity to see large groups of heroes going at it against each other in live action and a very easy way to intertwine the universes.