Specific fandom ideas
1. Two created characters, sorta like how Harry/Hermione/Ron had other friends that helped, the books never really discussed Sirius/Lupin/Lily/James's other friends, perhaps they had some, it's their 7th year, the dark lord is a serious threat, etc
2. I use a character I created that dated Sirius in school, someone else roleplays with Sirius during their pre-fall of the dark lord days (nothing sexual obviously)
1. It's years later, canon characters are no longer at camp, a group or two demigods must go on a quest (various quest ideas)
1. Before the fall of the time lords, the doctor's cousin foresaw what was to come and snuck away to earth, in the last 80s/early 90s and runs into Jack Harkness, a brief affair leads to the birth of a child that is hidden away to protect her from the fall of the time lords, when she turns 18 Jack is instructed to find her and open the locket that reveals her true identity.
2. Something entirely original from the last regeneration of the time lord (#13) and a companion