Negasonic Teenage Warhead appears in the films to have the powers of Cannonball from the X-Men comics.
Why was the character changed so much? Normally Negasonic Teenage Warhead has telepathic/pre-cognitive powers.
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Cannonball was originally intended for the movie because, to quote Tim Miller
"In the original script the action in the third act was great, but it was just Deadpool and a lot of guns
But they were worried about him being perceived as a hick:
"We thought about Cannonball, but he would’ve been a stupid hick character, whereas the guys wrote Negasonic as this deadpan goth teen, which was a great angle.
They choose the character based in the name, and spliced in the powers:
I went through the list of Marvel characters ... And at the end of that list was Negasonic, which I just thought was a freaky, funny name.
"We changed her powers because we thought it was funnier"
In a Q & A podcast the screenwriters Paul Wernick and Rhett Reese admitted that they only wanted to include the character because of her name and that they changed her powers because it was funnier.
At around 29:30 they answer the question as to the reason for the character being in the movie,
Negasonic was there because Tim [Miller] wanted some good guy super powers in the movie
And at 29:42 they answer the question as to why they wanted her,
Q: Why her? Why did you choose Negasonic?
A: Because of the name.
Then they talk about how they changed her powers, and how she was a mind reader or was able to look into the future, 29:50
In fact we changed her powers, we made her into a literal warhead because we thought that was funnier.
There is a lot of other great stuff in the podcast if you have time to listen to it.
And this on how they had to get permission from the studio,
It was the one thing we needed Marvel’s actual approval on, that they had to reach out for. Tim [Miller] has a relationship with [Marvel President] Kevin Feige and I think he went straight to Kevin because all the lawyers, you know, it gets messy with the lawyers.