Often, baby Killua in fanfiction is presented as having a strong bond with certain characters from an early age. His loyalty and protectiveness, which are traits that are more prominent as he grows older, are sometimes hinted at in his childhood. He can also be quite endearing, with his small - child clumsiness and his determination to learn new things.
Another theme is friendship. Killua is used to a certain type of friendship from his own story, and when he meets RWBY characters, new bonds are formed. It's about how he adapts to their group dynamic and vice versa. For example, his relationship with Ruby, who is very different from his friends back home.
Adventure is also very common. The OC usually gets involved in Killua's dangerous and exciting world of being a Hunter. They might be on a mission to find a rare item or defeat a powerful enemy. And there's often a theme of trust - building. Killua has to learn to trust the OC and vice versa.
One popular theme is their friendship. Their bond is very strong in the original work, and fanfics often expand on it, like how they rely on each other in dangerous situations. Another theme is adventure. Since they are both on a journey in the original story, fanfics may create new adventures for them, such as exploring uncharted lands.
You could write about Killua's friendships. For example, his relationship with Gon and how they support each other through their adventures. This would be a positive and in - line - with - the - spirit - of - the - story type of fanfiction.