One common theme could be adventure. Since it's set in an 'underground' setting, there might be elements of exploring the unknown, facing challenges and obstacles. Another theme might be the relationship between rex and the devilartemis characters, which could involve friendship, rivalry or even some form of cooperation.
I'm not sure specifically. You could try searching on fanfiction - specific websites like Archive of Our Own or FanFiction.net. They have a large collection of fanfictions and might have something related to Zarbon.
One possible plot could be that Naruto is somehow magically transformed into Zarbon. He then has to deal with Zarbon's powers and the reactions of the people in Zarbon's world. He might struggle to adapt to Zarbon's different fighting style and the social norms of that world while still maintaining his own Naruto - like determination to protect those he cares about.
Some Zarbon x Dodoria fanfictions explore a what - if scenario. For example, what if they were not under Frieza's control? In one such story, they break free and try to build their own empire. It shows a different side of them, not as minions but as leaders. They have to deal with all the problems that come with ruling, and their relationship evolves in this new context.
There could also be a fanfiction where Zarbon, after being defeated by Goku, decides to train under him. Zarbon is humbled by his loss and realizes that Goku has a lot to teach. Goku, being the kind - hearted Saiyan he is, agrees to train Zarbon. This story could show Zarbon's growth as a fighter and his transformation from an enemy to an ally of Goku.
A common theme is the exploration of their loyalty to Frieza. In many fanfictions, their actions are driven by their loyalty or their struggle with it. Another theme is the power dynamic between them. Since Zarbon is more powerful in some ways, and Dodoria is physically strong, this power play is often a part of the story.
Well, I'm not that familiar with specific 'rex in the underground devilartemis fanfiction' stories. But you could try searching on popular fanfiction websites like Archive of Our Own. They usually have a wide variety of fanfictions and you might be able to find some good ones related to this there.
One interesting 'Dragon Ball Z Zarbon' fanfiction could be about Zarbon's backstory before he joined Frieza's army. It might explore his home planet, his family, and what led him to become such a loyal follower of Frieza.
One possible plotline could be Zarbon's struggle to balance his duties while taking care of his pregnant wife. Maybe he has to face threats to his family from his enemies.
One more plot could be that Zarbon time - travels. He goes back in time and meets his younger self. He tries to change some of the events in his past that led him to be a somewhat villainous character. But in doing so, he risks changing the entire future of his race and the universe he knows.