Well, for example, Natsu's fire - dragon slayer magic could potentially interact with a heat - based alien form from the Omnitrix. Maybe when he's in that form, his fire magic becomes even more intense or has a different quality to it. He could use his dragon slayer skills to control the heat in a more precise way than the alien form normally allows.
Well, Ben is obviously one of the main characters as the story is centered around him and the Omnitrix. Gwen might also be a main character as she often plays an important role in Ben's adventures, providing support or using her own magical abilities to help out. Grandpa Max could be another main character, offering his wisdom and experience to guide Ben through various situations.
What if the Omnitrix was actually a gift from a legendary Pokemon? In this fanfiction, Ash is chosen to receive it because of his pure heart and strong will. However, using it comes with a price. Every time he transforms, it drains his energy, and he has to find a way to replenish it. This could involve seeking out special items or places in the Pokemon world. His friends would have to help him manage this new challenge.
One interesting storyline could be about Ben exploring new alien forms that were previously unknown in the official series. Maybe he discovers a form that has unique powers that he has to learn to control quickly to face a new and powerful enemy.
One great idea for a Pokemon Ash and Omnitrix fanfiction could be Ash finding the Omnitrix in the Pokemon world. Maybe it fell through a dimensional rift. He could use it to transform into different Pokemon - hybrids of Pokemon and the alien forms from the Omnitrix. This would give him a whole new set of powers and strategies to use in his Pokemon battles and his adventures.
In this fanfiction, Naruto might use the Omnitrix to gain various alien forms' powers. He could use those powers to fight stronger enemies in the Naruto universe, like the Akatsuki members. Maybe he would use the powers for stealth or super strength depending on the alien form.
One possibility is that the story could explore how Gwen's personality changes when she has the Omnitrix. She might become more confident or more responsible. Also, there could be new villains who are attracted to the Omnitrix now that Gwen has it. She would then have to face these threats and protect the Omnitrix from falling into the wrong hands.
Hermione would be extremely curious. She would immediately start researching it, trying to figure out its mechanics and where it came from. She might even try to find a way to improve it or adapt it to the wizarding world's technology.
Well, perhaps the Omnitrix gets mixed up with the magic in Hogwarts. It starts malfunctioning and instead of the usual alien forms, it gives Harry forms of ancient wizards. He has to figure out how to control these new forms while also dealing with the mystery of why the Omnitrix is acting this way. And he might learn new spells and magic from these wizard forms.