@Mai-koe You can ask, but some fanfic concepts are not always going to be unique. Being an anime character's sibling, parent, or friend, reincarnating into the body of an already existent character, or retelling the story from a character's perspective. as long as you intend to write your own unique story, then there is no need to feel bad. If you intend to take ideas directly, you can ask and see if they are alright with the use of those ideas, or maybe make an introduction chapter to the story saying you got several ideas from this other person.