In an interview with Collider, Deadpool 2 writers Rhett Reese and Paul Wernick revealed that supporting characters Negasonic and Colossus will be featured in at least some capacity.
What worked about the characters is that they were only supporting roles in Deadpool, which meant that they weren’t overexposed in the story. Plus, their conflicting senses of humour created an ironic clash: Negasonic was Hollywood’s idea of what emos are, while Colossus’ boy-scout personality juxtaposed his tough appearance. Plus, Deadpool didn’t feature any other supporting characters from the X-Men film series, which meant it felt small and unique, which really gave Deadpool a sense of self-identity. If Deadpool 2 is going to be just as good as the first one, Deadpool needs to remain the focus, which means that Negasonic and Colossus need to remain as supports. This means that there’s probably going to be only limited scenes at Xavier’s School for Gifted Youngsters, as per before. What works so well about Negasonic, Colossus and Deadpool as a trio is their setup as a comedy troupe: Negasonic and Colossus are Deadpool’s “straight men”, which make Deadpool only funnier. And it’s the moral differences that are at the centre of their relationship: Colossus and Deadpool are totally opposed when it comes to empathy. Perhaps that’s something that Deadpool 2 could explore.