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.
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.
One idea could be that a character from Fairy Tail finds the Omnitrix and has to learn how to use it without getting overwhelmed by its powers. Maybe they initially use it for simple tasks like getting out of tight spots during a guild mission, but as they get more proficient, they start using it in more complex battles against powerful foes.
In this fanfic, the relationships among the Omnitrix aliens - as - humans would be full of surprises. The competitive relationships between some aliens might turn into friendly rivalries. Take Diamondhead and Four Arms, for instance. As humans, they might compete in sports rather than in battles. Their relationships with humans who don't know their true identities would also be interesting. They would have to constantly lie to maintain their cover, which could lead to feelings of guilt and isolation. But at the same time, they could form deep and meaningful relationships with those who accept them for who they are, despite their secrets. This would create a web of relationships that are both complicated and fascinating to explore.
The story could start with Gwen unexpectedly getting the Omnitrix. Then, she might start exploring the different alien forms she can transform into. She'd likely test out their powers in various situations, gradually getting a handle on how to use them effectively.