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 great Ben x Kevin fanfiction could be about their adventures in a parallel dimension. In this story, they might face new aliens and challenges that test their friendship even more. Maybe they find a powerful alien artifact that only works when they combine their unique abilities, leading to a deeper understanding of each other.
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 interesting storyline could be a female version of Ben having to deal with different alien cultures in the Omniverse in a more diplomatic way than Ben usually does. For example, she might use her charm and intelligence to resolve conflicts between warring alien species on a planet instead of just using brute force with the Omnitrix transformations.
One interesting concept could be creating new alien forms for Ben's Omnitrix that are unique to the fanfiction. Another might be exploring different relationships between Ben and his OC (Original Character) friends or foes. For example, an OC could be from a planet that has a very different culture from what Ben has encountered before, leading to interesting interactions and storylines.