They would probably be very proud. Despite their differences, both Hinata and Neji are highly skilled members of the Hyuuga clan. For the clan, watching them fight in the fanfiction would be like seeing the best of their combat techniques and kekkei genkai on display. They might also be analyzing their moves to see how they can further improve the clan's fighting style.
Well, the Hyuuga clan has certain expectations and standards. Their presence during the Hinata vs Neji fight in the fanfiction would make both fighters more conscious of these. Hinata, who has always been in Neji's shadow in the eyes of some in the clan, would be trying extra hard to show that she can be a great Hyuuga. Neji, who has always been seen as a strong representative of the clan's old ways, would be fighting not just to win against Hinata but also to maintain his status among the clan. Their fight is thus shaped by the clan's overarching influence.
In a Naruto pranks Hyuuga fanfiction, Naruto might use his typical mischievous tactics. For example, he could use his shadow clone jutsu to create chaos around the Hyuuga compound. Maybe he'd make clones that pretend to be important visitors and then act all silly to embarrass the Hyuuga. It would be a really funny scenario as the serious Hyuuga clan tries to deal with Naruto's antics.
There's a 'Harry Hyuuga fanfic' that focuses on his relationship with the Hyuuga members. In this story, he forms a deep bond with some of the younger Hyuuga and they go on adventures together, facing both magical and non - magical challenges.
It is not ethical. Hinata Hyuuga is a character, and creating fanfiction that involves abuse goes against the spirit of respecting the character and can also be disturbing to other fans who love the character for her positive traits.
Naruto could prank the Hyuuga by changing the signs on their training areas. So instead of the correct instructions for their Gentle Fist training, there are completely wrong and silly ones. This would disrupt their training sessions and lead to some really funny moments as the Hyuuga try to figure out what went wrong.
Archive of Our Own (AO3) is also a great place to look for Harry Hyuuga fanfics. It has a well - organized tagging system, so you can easily search for the specific fanfics you're interested in.
In this kind of fanfiction, Naruto might gain access to the Hyuuga's unique fighting techniques. For example, he could start learning the Gentle Fist style. This would make him an even more formidable ninja. He might also be exposed to the Hyuuga's clan politics and traditions, which could be a whole new world for him compared to his life as an orphan in the original story.