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.
Bulma could be another character paired with the reborn Broly. She might be the one to help him understand the modern world if he is reborn in a different time period. She could use her intelligence to create devices that help Broly control his power better or she could just be there to provide some comic relief in the midst of all the intense Saiyan drama.
He could team up with Luffy. Luffy is always open to new allies, and Broly's power would be a great addition to the Straw Hat crew. They could go on adventures together and face powerful enemies.
Goku could be a likely ally. Despite their initial conflicts, Goku has a way of seeing the good in people. He might approach Broly and convince him that they can work together to win the Tournament of Power for their universe. Their combined strength would be a formidable force.
In a crossover with One Piece, Luffy could be an ally. Luffy is known for his open - mindedness and ability to befriend almost anyone. He would see Broly's power and might try to get him to join his crew or at least fight alongside him against common enemies. They could have some really fun interactions, like Luffy trying to make Broly laugh or share his love for food with him.