First off *SPOILER ALERT*. If you haven't seen Deadpool don't read this article. In my opinion Deadpool missed out on a big opportunity to have Vanessa become Copycat in this film. Maybe they want to pace the development of Vanessa first but here's what I'd have done if I was in control.
During the holiday sex montage early in the movie, Wade turns off the light to start the "session". Vanessa confesses her fear of the dark. This fact isn't important until the end of the film during the final battle sequence. Except this time instead of Deadpool and Ajax getting right to dancing, Ajax injects Vanessa with the mutant ability influencing serum. The battle pursues as it did originally. After the ship explodes and Colossus and Negasonic are searching through the rubble, Colossus hears Vanessa's fearful screams and uncovers oxygen deprivation tank to reveal Vanessa who is now blue. Wade runs over to help her out. Vanessa's scratches then start to heal and she embraces Wade.
This would lead to an interesting sequel with both having powers and Vanessa eventually discovering her powers extents and her shapeshifting. If her powers aren't obtained in the sequel at least, I'll be slightly disappointed. It will still be good I'm sure though! Tell me what you think would have been better in the movie.