Rosalie could be an interesting character to include. She has a complex personality and her relationship with Jasper, as they are both part of the same coven, could be explored further. Emmett as well. His strength and jovial nature could provide some comical relief in a story that might otherwise be quite serious with Alice and Jasper's more intense relationship.
I think Carlisle would be great. His wisdom and leadership within the coven could play a role in the fanfiction. For example, if Alice and Jasper were facing a difficult decision, Carlisle could offer his advice. Esme too, as she is like the mother figure. Her kindness and nurturing nature could be a contrast to some of the more intense vampire traits of Alice and Jasper.
Edward could be a good addition. Since he has his mind - reading ability, it could create some interesting interactions with Alice's visions. Also, Bella could be included as she has a connection to both Alice and Jasper through Edward. Her human perspective could add a different dimension to the story.
Bella could also be a significant character. She might be the reason for their meeting in some stories. For example, if Bella is in trouble, Alice and Jasper could come together to protect her. Her human nature can contrast with the vampire nature of Alice and Jasper, creating an interesting backdrop for the story.
Alice could be a great addition. She could be Bella's best friend who is always trying to play matchmaker between Bella and Jasper. She has this uncanny ability to sense when things are going well or not so well between them and gives her two cents whether they want it or not.
Well, Kikyo is an obvious choice. Her connection to both Inuyasha and Naraku is deep. Her presence could create a lot of tension, especially since Kagome is often compared to her. Shippo could also be in the fanfiction. His cute and innocent nature could contrast with the dark nature of Naraku. And Kaede, the old priestess, could offer her guidance and wisdom regarding the strange relationship between Naraku and Kagome.
Lois could be a great addition. Her relationship with Peter and her reactions to his antics with Quagmire can add a lot of drama. Also, Cleveland. His calm and collected nature contrasts with Peter and Quagmire's craziness. His interactions with them can lead to some really funny moments.
Jacob could be a great addition. He has a complex relationship with Bella in the original series. In fanfiction, his interactions with Charlie could be explored further. For example, how Charlie reacts to Jacob's wolf - shape - shifting abilities and his love for Bella.
Santana could be a great addition. She could be Rachel's best friend who gives her brutally honest advice about married life. Like when Rachel has a fight with Jesse, Santana would be there to say something like 'Girl, you need to put your foot down or he'll walk all over you.'
Edward could be included. He could be Emmett's rival in a sense, not in a romantic way towards Rosalie, but perhaps in a career or talent aspect. For example, if Emmett is into sports, Edward could be a really good athlete too. This creates some friendly competition between them. Also, Leah could be a character. She could be Rosalie's coworker and through their friendship, Leah could influence Rosalie's relationship with Emmett in different ways.
Raphael could be a character to include. He might initially be skeptical or even opposed to Leonardo's relationship with Karai due to her past actions. His presence could add some internal conflict within the group. Also, Shredder could be in the background as a looming threat, which could heighten the tension in their relationship as they try to keep it a secret from him.
Steve Trevor could be a great addition. His relationship with Wonder Woman adds an element of human connection. He could be caught in the middle of the conflict between Wonder Woman and Cheetah, or he could play a role in helping Wonder Woman understand Cheetah better. For example, he might have some insights from his military experience that could assist in dealing with Cheetah.
Edward could also be included. His relationship with Rosalie as her adoptive brother and his complex feelings towards Jacob could create a lot of tension and drama. He could be a voice of reason or a source of conflict, depending on the direction of the fanfiction.