A common theme is family. Given their complex family situation in the original story, fanfictions often explore how Killua and Alluka's relationship affects and is affected by their family. Another theme is protection. Killua is very protective of Alluka, and this is often a central aspect in these fanfictions.
One great 'killua x alluka fanfiction' could be one where they go on a journey to find a cure for a strange illness that has befallen their family. In this fic, Killua's protective nature towards Alluka is highlighted, and their relationship evolves as they face various challenges along the way.
Gon in fanfiction can be very single - minded in his pursuit of goals, just like in the original. He might be a bit more naive at times, which can lead to interesting plot twists. Killua is often portrayed as having a complex emotional side. He struggles with his past as an assassin and his loyalty to Gon. His cool exterior may crack in some fanfictions, showing his true feelings.
One common theme is Alluka's power and how it affects those around her. Her powers are so unique in the 'Hunter x Hunter' universe that fanfiction often explores how different characters react to it.
In a Naruto world fanfic, Killua would bring a whole new dynamic. He has a dark past as an assassin which could either make him a feared or respected figure. His family's training has made him physically and mentally tough. In battles, he could use his electricity - based attacks in creative ways against ninjas. But he would have to learn about things like jutsus and the ninja villages. Also, his relationship with Gon in the HxH world was important for his character development, and in the Naruto world, he might find a similar bond with a fellow shinobi, which could change his perspective on things.
Yes, there are likely some fanfictions out there with such a concept. Fanfiction writers often come up with creative crossovers. You can try searching on popular fanfiction websites like Archive of Our Own or Fanfiction.net.
There's this fanfiction titled 'Alluka's Journey'. In it, Alluka goes on an adventure on her own, away from the main plot of 'Hunter x Hunter'. It delves deep into her powers and how she discovers more about herself while facing various challenges along the way. The story is beautifully written and really captures Alluka's essence.
He could form a rival - like relationship with Sasuke. Both are highly skilled and have a bit of a dark side. They might constantly compete to prove who's better in combat.