A third Deadpool movie is in the works. Illyana appearing in that would make sense since Colossus is already a character. If there's some sort of dimensional travel involved that moves Deadpool into the MCU, then Illyana could be involved.
There needs to be supporting characters for Deadpool. If they can't use the regular cast of the X-Men movies, then adding in actors from New Mutants would be a good alternative.