One idea could be to crossover Yugioh Zexal with another popular anime like Naruto. The characters from Zexal could find themselves in the Ninja world and have to adapt to new rules of dueling. For example, they might have to use their duel monsters in combination with ninja techniques.
You can start by choosing an unexpected crossover partner. For instance, if you cross it with a fantasy series like Lord of the Rings. The duelists from Zexal could enter Middle - earth and their duel monsters could interact with the magical creatures there. The goal could be to use dueling to stop an evil force that threatens both worlds.
I don't know which specific game or card game Yu-Gi-Oh zexal refers to. If I can provide more information, I will try my best to answer your questions.