re: Rewrite the endings of your favorite shows/movies

Avengers 2 - Ultron kills everyone in the city before trying to launch it into the air. The moment when the Twins approach him in the church happens now rather than earlier (they just meet him in a different manner, or you could even use the earlier meeting as foreshadowing). They realize he killed all their fellow countrymen and turn on him. The battle alerts the Avengers who show up as Ultron is whooping the crap out of the twins and help turn the battle. Ultron brings in his army of bots for some awesome action scenes, Quick gets offed by Ultron as the Avengers look to finally win, Witch goes psycho on his bots and him. At some point Ultron announces to the world that Tony Stark created him. Avengers win but Tony decides to hang up the suit. Thor leaves for his vision shite (aka Thor 3), and Cap still believes the world needs the Avengers and makes the new team. Movie ends with different world governments saying the Avengers need to be regulated along with people of interest.

Ultron looks like a badass, sets up Civil War much better, and for the most part things are still status quo from how the movie actually ended.