An adventurous friend for the main character. This friend could join the main character in their journey through different lives, or at least in the exploration of memories from past lives. They could be the comic relief as well as a support system for the main character.
A mysterious fortune - teller could be added. This character could have some knowledge about the main characters' past lives and give them cryptic messages that drive the plot forward. They could be an old and wise figure, adding an element of mystery to the story.
A rival from a past life. Maybe someone who had a conflict with the main character in one of their previous lives and now resurfaces in the 19th life, causing trouble and forcing the main character to face unresolved issues from the past.
The main character is a girl who can remember her past lives. And there is also a male character who has a deep connection with her.
I'm not sure exactly who the main characters will be in the side story. But it's likely that the main character from the original '19th life' story will play an important role.
One idea could be exploring a different timeline where the main characters meet in a completely unexpected place in one of their past lives. For example, in the 5th life, instead of the normal setting, they meet in a far - off land during a war.
Adding a character like Amanda Waller would be interesting. She could be pulling the strings behind some of Peacemaker's missions, adding a layer of mystery and political intrigue. Or even bringing in a character like Green Arrow for a brief encounter to see how their different ideologies clash.
A young witch named Luna, not the same as Luna Lovegood, who has the power to control the weather within the Hogwarts grounds. She could be a new friend for Harry and get involved in his adventures. Another could be a wizard named Orion, who is an expert in ancient runes and helps Harry decipher some hidden messages. There could also be a half - elf named Elara, who works in the Hogwarts kitchen and has a special connection with the magical plants there.
Hermione Granger from the 'Harry Potter' series could be an interesting addition. She could use her magic knowledge to help the Ghostbusters understand the more esoteric aspects of the ghosts they're dealing with. Maybe she could even develop spells that work in tandem with the Ghostbusters' technology.
A group of time - traveling historians. They accidentally cross paths with Addie during one of their trips. They are fascinated by her and want to study her unique situation. This could bring in elements of different time periods as they try to figure out how Addie has survived for so long. Their presence could also put Addie in danger as they might accidentally expose her to the wrong people.
Remus Lupin would be a great addition. He's another one of James' friends. He could offer them wise advice, especially when it comes to the more difficult aspects of their relationship or the challenges of living in the wizarding world. He might also help them out during full moons.
A character like a Pokémon whisperer who can communicate with Pokémon on a deeper level than anyone else. This person could teach Ash new ways to bond with his Pokémon. Another could be a wandering minstrel who travels around the Pokémon world, singing tales of legendary Pokémon and has clues about a powerful Pokémon Ash is seeking.
Lex Luthor would be an obvious choice. His complex relationship with Superman could add a lot of drama. Also, Supergirl could make an appearance. She could team up with Superman to fight a common enemy while also bonding with Lois as another strong female character.