Negasonic Teenage Warhead was put in Deadpool basically by virtue of a) being a character that wasn't vital to the X-Men movies and b) having a really cool name. So it's no surprise that she could have looked like pretty much anything. Concept artist Joshua James Shaw posted a bunch of work from Deadpool, showing off the title character's suit and the many, many possible looks for NTW. Unsurprisingly, Deadpool's outfit looked basically the same from conception to execution:
All concept art: Joshua James
Negasonic, on the other hand, was given a bunch of options.
Honestly, given how stereotypically "sexy goth" a lot of these designs are, it's a good thing that they went with a more "First Class" sort of X-Men design for her in the end.