Firstly do you want to introduce a new OC or do some character development for a current one
If your introducing a new character then you should put a/the character in a dangerous situation that can show the new OC’s capabilities and teasing a secret the OC doesn’t know to encourage people to want a part two
If you are doing character development for a current OC then you’re gonna want to make a villain. This can be a person or a disease or something like that. Then the OC has to face this and learn a lesson from the experience he/she has