Hello:
My opinion is that the author is the one having the power of the story because the story comes from the author, but I think that suggestions are always welcome even though some will not make it into the final edit, but that doesn't mean they were not considered.
While using a character that belongs to someone else, the author and the character's owner must be a bit of careful, because even if the added character can add more depth to the story, the author must realize that the character's owner does have plans for the character, so the author cannot just kill it or maim the character without the owner's consent. This can make thing complicated perhaps, but if there is an angreement, then things should not turn difficult and it could just add more fun.