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.
Ben Tennyson is of course one of the most popular. Since he is the original wielder of the Omnitrix in the canon, he often takes center stage in fanfiction. Fans love to write about his new adventures and how he continues to grow and develop his skills with the Omnitrix.
In some Omnitrix fanfiction, there are crossovers with other universes. For instance, a story where the Omnitrix wielder ends up in the Star Wars universe. They have to use their alien forms to survive in this new and very different environment, facing off against Sith Lords and using their powers in creative ways to interact with the Force.
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 idea could be exploring a new alien form that wasn't in the original series. Maybe an alien with the power to control time in a unique way. Another idea is to have Ben go on a mission in a completely different galaxy, facing new challenges and enemies.
Obviously, Ben is a must - include character as he is the main user of the Omnitrix. Gwen can also be a great addition as she has her own powers and can provide support or be a source of conflict. Grandpa Max is another good character, with his experience and knowledge.
In this fanfiction, Gwen might use the Omnitrix to fight new and unexpected villains. She could transform into various alien forms and discover new powers within each transformation.