To write them well, you need to understand their backstories. Withered Bonnie has been through a lot, maybe he's seen better days. Toy Chica is newer but might feel out of place among the older animatronics. Use their backstories to create believable interactions. For example, Withered Bonnie could be jealous of Toy Chica's newness at first, but then grow to appreciate her uniqueness.
Another plot might be a mystery - solving one. There could be something strange going on in the pizzeria, like missing parts or strange noises. Withered Bonnie and Toy Chica team up to figure out what's behind it all. Maybe they discover a long - forgotten secret about the place that changes everything for them.
The plot could be a love story in a really unique way. Withered Bonnie, with his worn - out appearance, might be initially shunned by Toy Chica. But as time goes on, she sees his kind 'heart' (if animatronics can have such a thing). They could then have a relationship that defies the expectations of the other animatronics. For example, they might share secret hiding places in the pizzeria and have little 'dates' where they just sit together and exchange their experiences.