One common theme could be protection. Just like a mother would protect her child, the Erza - like character in the fanfic might protect Naruto from various threats in the Naruto universe.
I'm not sure specifically which 'erza narutos mother figure fanfic' are the best. You could try searching on popular fanfic websites like FanFiction.net or Archive of Our Own. They usually have a wide range of fanfics related to Naruto and might include ones with an Erza - like mother figure.
No. Slavery is an inhumane and immoral concept, and applying it to fanfiction characters from a series like Naruto, which has positive values, is completely inappropriate.
I'm not entirely sure as it's a rather specific and perhaps not so common fanfiction concept. It might be about Harry from the Harry Potter series being related in some strange way to Naruto's'splitting' concept, like a clone or some other form of division. But without reading it, it's hard to say for sure.
I'm not sure specifically as this is a rather unethical and inappropriate concept for fanfiction. In the Naruto universe, Hinata and Naruto have a relationship based on mutual respect and admiration, not slavery.
I'm not aware of the popularity of this specific fanfic. There are so many Naruto fanfics out there that it's hard to say whether this one has gained a large following.
Well, it could be that Minato is somehow transformed into an animal and Naruto finds him. Naruto, being kind - hearted, takes him in as a pet. And then there could be all sorts of adventures as Naruto tries to figure out how to turn Minato back, while also dealing with the normal Naruto - world problems like fighting bad guys and training.
I'm not sure of a specific 'minato is narutos pet' fanfiction off the top of my head. However, you can try searching on popular fanfiction websites like FanFiction.net or Archive of Our Own. They have a vast collection of Naruto - related fanfictions, and you might be able to find one with such an interesting premise there.
One common theme is family acceptance. Since in the original Naruto story they are not siblings, the fanfictions often focus on how they come to terms with being related. Another theme could be the change in their relationship dynamics. For example, Sakura might become more protective of Naruto or vice versa. Also, the impact on their friends and the ninja world in general is often explored, like how their new family bond affects the way others see them and interact with them.
One possible plot could be that Hinata is magically turned into Naruto's pet. Naruto, being his kind-hearted self, takes great care of her. He might try to figure out how to turn her back while also having all sorts of adventures with her in this new form. Maybe they go on missions together with Hinata using her unique abilities in her pet form to help Naruto out.