Scarlet Witch was created by Stan Lee in the Marvel Comics as a mutant and daughter of Magneto. She and her brother eventually joined up with the Avengers in the comics and served as initial members of the team.
Even though Scarlet Witch was not seen much as a member of the team in the animation, however the only two shows that she was seen in as an Avenger before the upcoming Avengers Age of Ultron were the Marvel Super Heroes Show of the 60's, The Avengers United They Stand and the Superhero Squad Show, As of right now since she is making her appearance in Avengers Age of Ultron, there are abosolute reasons on why she mush survive against Ultron in the final battle and for why Elizabeth Olsen must return to reprise her role for future Marvel Cinematic films after that
1. Since Scarlett Johansson's Black Widow was the only female hero from the line up of Avengers from the first film, Scarlet Witch's allignment to the team throughout the films should help keep Black Widow from being the only female surrounded by men.
2.With Scarlet Witch posessing the powers of telekinesis, mind control, and so much more, she is way to awesome of a superhero to be wasted
3. With her powers she could be a really useful ally to the Avengers in their battle against Thanos
4. As an inhuman, Scarlet Witch should live to tell the Avengers as much as she knows about people like her and her brother Quicksilver
5. Even though the rumors of her brother's death are yet to be found out first hand, a rumored source from Disney confirms that she and the Vision will survive to jointhe Avengers in their new rooster for future Marvel Cinematic films, at least we hope its true
6. Her involvement in the upcoming Captain America Civil War movie should make the plot of the film more effective on the government's intended control over the heroes
7. Tony mentions to Steve Rogers that Wanda showed Thor something interesting which may happen after the Avengers defeat Ultron indicating that she will survive and return for future MCU installments
8. Elizabeth Olsen and Aaron Taylor Johnson are reported to have multipicture deals so even if Quicksilver dies in the film, Scarlet Witch will still survive and Olsen will return for more MCU films after all having a multi-picture contract means more than 1,2,3 and 4 films
9.And as a member of the Avengers, Wanda is more of a team player of the league of heroes than her brother Pietro
While we wait for the Avengers Age of Ultron to premier in theaters, we hope to get confirmation on Elizabeth Olsen's return for Captain America Civil War sometime soon before April 26th and that should help bring our confidence back up as well so stay tuned and keep watching for anymore updates on the movie that hit the net