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.
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 possible fanfiction could be about Rex's adventures in a new, uncharted area where he discovers a strange new type of E.V.O. He has to use his powers in creative ways to deal with it and learn more about this new threat or ally.
In a post - apocalyptic world, Generator Rex was on a mission. Rex was also exploring the same area. They both heard rumors of the Black Knight. When they finally met the Black Knight, he was not what they expected. The Black Knight had a code of honor. Rex was intrigued and Generator Rex was curious about his technology. They started to exchange knowledge. The Black Knight taught them some ancient combat techniques and in return, Generator Rex shared his knowledge of advanced machinery. Rex learned a lot from both of them and they became friends, vowing to protect the world together.
Fanfiction about Rex and Six could also involve a mission where they are sent undercover. They have to pretend to be ordinary people in a community that is on the verge of being overrun by E.V.O.s. They need to figure out how to stop the E.V.O.s without blowing their cover, and in the process, they learn more about each other's personalities outside of the battlefield.
Naruto might try to befriend the Indominus Rex. He's always been one to see the good in others, even enemies. So he'd probably approach it with an open heart. Sasuke, on the other hand, would be more cautious and likely see it as a threat to be eliminated if necessary.
A possible plot could be that the Black Knight is wrongly accused of a crime. Rex and Generator Rex are initially tasked with capturing him. But as they get to know him during the chase, they realize he is innocent. So, they join forces to find the real culprit who is actually a powerful and corrupt figure trying to frame the Black Knight to gain more power in the post - apocalyptic world.
I'm not entirely sure specifically what '6 underground fanfiction' is about as it could vary greatly. It might be fan - created stories based on the movie '6 Underground'. Maybe it focuses on the characters' backstories that weren't fully explored in the film, or it could be about alternative adventures or relationships among them.
In fanfiction, Rex's character often develops from a battle - hardened clone to a more empathetic and wise figure. He learns to trust his new allies in the Rebels, like Ezra and Kanan. His past experiences in the Clone Wars shape his decisions in the Rebellion, making him more cautious yet determined.
Rex found himself in a strange, swirling vortex of light. Suddenly, he was thrust back in time to the prehistoric era. He stared in awe at the dinosaurs roaming freely. Rex knew he had to be careful not to disrupt the natural order. He hid behind a large boulder as a T - Rex passed by, its massive feet shaking the ground. Rex started to wonder how he could find a way back to his own time while also making the most of this once - in - a - lifetime opportunity to witness history up close.