The strongest in Hunter x Hunter is a matter of debate. It could be Gon when he unlocks his full potential, or maybe it's a character like Zeno Zoldyck with his legendary status and skills.
Chrollo's brother in fanfic can be a really interesting character. He might have similar powers or a completely different skill set. For example, he could be a master of a different type of nen ability that contrasts with Chrollo's. This could lead to some really cool plot lines where they either team up or are at odds with each other.
One idea could be a story where Pitou's brother is a human-turned-Chimera Ant. He has retained some of his human memories and is constantly in conflict with his new Chimera Ant instincts. Pitou tries to help him come to terms with his new existence while also dealing with the Hunter x Hunter world's chaos.
An important aspect of creating an OC for a Hunter x Hunter fanfic is their appearance. You can take inspiration from the diverse and unique character designs in the anime. They could have a distinct hairstyle, like long, flowing hair with braids that are common in some regions of the Hunter x Hunter world. Their clothing should also reflect their personality and role. If they are a stealthy type, they might wear dark, form - fitting clothes. In terms of personality, they could be a mix of traits. Maybe they are initially shy but become bold when it comes to protecting their friends, much like some of the existing characters in the series.
The Hunter x Hunter manga is unfortunately not complete. The author has faced various issues that have led to intermittent breaks and an unresolved storyline.