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.
One character to include is Rin. Since she has a connection to Sesshomaru, who has his own relationship with Naraku. Her kindness might also have an impact on Naraku if he is in a situation where he is changing. Totosai, the swordsmith, could also be included. He could create a weapon or item that has something to do with the relationship between Naraku and Kagome, for example, a weapon that can only be wielded if they work together.