It's hard to say for sure, but Naruto could be a very strong contender. His ability to create countless clones and his massive chakra reserves would give him an edge. Plus, his Sage Mode and later forms are extremely powerful. However, Ging from Hunter x Hunter also has a lot of mystery and power surrounding him. He might be able to outwit Naruto in a battle of wits. But then again, Sasuke with his Rinnegan and Sharingan abilities could also be very formidable in this crossover.
One way is through intense training. In fanfiction, a character might undergo extreme physical and mental training regimens that push their limits. For example, Gon could train in isolation with more powerful nen masters, learning new and advanced nen techniques that weren't shown in the original series. This could include more complex manipulation of his aura, like creating multiple types of nen beasts for different purposes.
One popular theme is the underdog rising to power. Many fanfictions focus on characters who were initially overlooked or weak in the original series, like Kurapika's side characters from his clan. They might start with very little power but through determination and some unique circumstances, they become extremely powerful. This gives readers a sense of satisfaction as they see the 'little guy' succeed.
The character crossovers are also really cool. Imagine Gon and his friends hunting a Rathalos! It brings a fresh perspective to both franchises. Also, the fanfic often explores new environments that are a blend of the two universes, like a forest filled with strange creatures from both Monster Hunter and Hunter x Hunter worlds.
Hisoka is extremely popular in fanfiction. His unpredictable and somewhat insane nature makes for great plot - driving elements. Fanfictions might explore his motives more deeply or put him in situations where he has to interact with the main characters in unexpected ways. Additionally, characters like Illumi also gain popularity as his complex relationship with Killua can be explored in many different and fascinating ways in fan - created stories.
One really creative piece is 'The Blending of Two Worlds'. In this fanfiction, the worlds of Naruto and Hunter x Hunter are not just combined in terms of characters meeting. Instead, the story creates a whole new world that has elements of both, like a new power system that is a blend of Chakra and Nen. The characters have to navigate this new world while also dealing with the politics and power struggles that come from both original series.
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.
Leorio and Yang would be quite a combination. Leorio's medical knowledge could be useful when Yang gets injured in battle. And Yang's brash and energetic personality could pull Leorio out of his more serious and studious mode. They could also have some funny exchanges about money and values, as Leorio is often concerned about finances while Yang is more about the thrill of the fight.
I think Killua and Ziva would make a great pair. Killua has his assassin background and lightning - fast reflexes, and Ziva is a former Mossad agent with her own set of combat and investigative skills. They could have some great action scenes together and also share their experiences of growing up in a world of danger and secrecy.
Kurapika could also be a likely character. Given his intense and often self - sacrificing nature in his pursuit of revenge, his friends would want to take care of him. Leorio, in particular, would be the one to bring him breakfast in bed. It could be a moment of respite for Kurapika from his heavy burdens, and a chance for some heart - to - heart conversation with his friend.