Re: Deadpool Spin-off Confirmed - Part 1
Colossus was in the script for two reasons.
1. To make it apparent that even though the Deadpool film wouldn't directly tie in to the continuity of the X-Men films, the X-Men do exist in Deadpools universe. His crack about Charles Xavier being a creepy bald guy was great.
2. He's a moral compass for Deadpool. He's trying to nudge him in the direction of altruism.
I think the writers chose Colossus to fit that role simply because he got the shaft in the X-Men films and maybe they are fans of the character.