You probably can't count on the accuracy of this, but supposedly X-23 and Colossus were added to New Mutants as part of the reshoots. X-23 is to be a major character, while Colossus was part of a (cut) subplot centered around him looking for his sister. However this is a part of the dubious "insider report" claiming that NM had been cancelled, which seems to be contradicted by other information (IE the insider claiming Fox was dissatisfied with both, even though all official word has been to the contrary).
Article here.