There are so many fine lines to walk with a project like this. You'd want to add moments and sequences that are the tried and true elements of the source material but you also don't want to create a synthesis of remixed scenes we've already seen a million times before.

I'd open with the third act of the Wanda v. Dr. Strange (Avengers). Big fight, have Avengers be the aggressors. Wanda deals with the team handily but Strange gets the drop on her. She is rescued by a powerful mysterious figure: Magneto, her father.

Magneto's motivation is obvious but his choice to act and intervene in superhuman affairs is particularly contentious to his closest friend, Charles Xavier. Prof X is seeing faultlines and troubling trends in society though. His students aren't ready to encounter a group like The Avengers. We meet X-Men in media res but in its infancy (just original team), mutants have always been among us, etc, etc.

Smarting from their beat down and trying to get to the bottom of the powerful, mysterious figure the Avengers uncover the origins of mutants. And Wanda is one. Exposition time. Same general explanation but I'd foreshadow a link to mutants and deviants to establish the X-Men as controversial. Avengers react, there's conversation, opinions are expressed. The mission to shut down Wanda just got a lot bigger.

In the end: Big fight! The Avengers program is disassembled after the loss as they now seem incapable of dealing with a burgeoning mutant scourge on society. The X-Men transform from a peaceful group of students to a para-military force through their battle with the Avengers and more importantly Magneto. In his use of deadly force during the battle (yes, somebody dies), Magneto is established as a villain and ensures Xavier's dream of peaceful coexistence is not an option. Magneto is wanted as a global terrorist and murder and retreats to his newly created lair, Asteroid M.