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.
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 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.
An old war veteran who has knowledge of some long - forgotten secrets related to the super - soldier serum. His knowledge could be crucial for Falcon and Winter Soldier in their fight against a new enemy that is also interested in the serum.
A character who is a master of shadow puppetry, but not in the normal sense. Instead of just creating simple shapes, they can create entire landscapes and armies of shadow creatures. This character could have a mysterious backstory and their own motives, which could either make them an ally or an enemy in the fanfiction.
An American wizard detective. He could be investigating strange magical happenings in the wizarding world of America. The Potter character could cross paths with him and end up teaming up to solve a big case.
One character that would be great to add is a time - traveling adventurer. This character could accidentally bring Billy and Mandy to different time periods. They could end up in the prehistoric era, where they meet dinosaurs and cavemen. Or they could be in the future, in a world full of high - tech gadgets that they don't understand. The time - traveling adventurer could then be their guide through these strange times, while also having his own motives and secrets.
A wise old owl could be added. The owl could be a guide for the dog, giving him directions when he's lost or advice on how to deal with difficult situations. Another character could be a friendly fox. The fox might play tricks on the dog at first but then end up becoming a loyal friend and helping him on his journey.
An angel guide could be added. This character could help the boy navigate the complex rules and wonders of heaven. Another character could be a mischievous spirit who often gets the boy into funny or challenging situations.
A psychic human. This character could sense the demons' movements and intentions before they attack. They could use their powers to help the Doom Slayer or other marines, for example, by warning them of an ambush or guiding them through a maze - like area filled with demons.