The adult baby bouncer chair can be a key element in a mystery in the fanfiction. Maybe it has some hidden compartments or powers that the characters are trying to discover. It could be left in a strange place with no explanation, and the characters have to figure out what it is and why it's there. This can add an exciting element of mystery and intrigue to the story, as the characters piece together the significance of this strange object.
One way is to use it for character development. If a character is forced to sit in an adult baby bouncer chair, it can show their reaction to a situation of helplessness or infantilization, revealing aspects of their personality like patience or impatience.
It can be used for comical effect. Picture a serious, no - nonsense character suddenly being placed in an adult baby bouncer chair and bouncing around. The contrast between their normal demeanor and this silly situation can create a lot of humor in the fanfiction. Also, it can be a part of a dream sequence, where the rules of reality are bent, and the chair represents something deeper about the character's subconscious desires or fears.