A more complex idea could be that the magic in the Fairy Tail universe somehow affects the Omnitrix, causing it to mutate and create new and unexpected alien forms. This would bring in elements of both worlds in a very unique way, and the characters would have to figure out how to deal with these new forms and the potential threats or benefits they bring.
You could write a 'Fairy Tail Fanfic' where the magic system is completely different. For example, instead of having different types of elemental and non - elemental magic, there's only one type of magic but different levels of mastery. Or a fanfic that focuses on the relationships between the guild members in a romantic aspect. Like a love triangle between Gray, Juvia, and a new character.
Ben Tennyson himself is a great choice. His ability to transform into different aliens offers a lot of potential for interesting interactions with the magic of Fairy Tail. For example, how would his Heatblast form interact with a fire - using mage from Fairy Tail? It could be a really cool dynamic.
A crossover with 'Naruto' could also be cool. Imagine the Fairy Tail guild members in the Hidden Leaf Village. They could participate in the Chunin Exams, and their magic would be a new type of jutsu. There could be battles between Fairy Tail mages and ninja, like Natsu vs Sasuke, which would be an epic showdown of fire - based powers.
One idea could be a crossover where the characters from Fairy Tail and RWBY team up to fight a common enemy. For example, they could face a powerful dark mage who has the ability to manipulate Grimm. The Fairy Tail mages could use their magic, while the RWBY teams use their weapons and semblances in an epic battle.
Well, in Ben 10 vs Fairy Tail fanfic, a really interesting crossover is when the power - up systems cross over. Imagine Ben 10's Omnitrix getting a magical upgrade from Fairy Tail's magic. Or the Fairy Tail mages getting some alien - like powers from Ben 10. Also, the cultural exchange between the two universes can be fascinating. Ben 10's modern - day Earth background compared to Fairy Tail's more medieval - like fantasy world can create some really unique storylines when they interact.
A good crossover idea is to have a character from Ben 10, like Gwen, use her magic to help the Fairy Tail guild in a difficult situation. Maybe there's a dark magic that only Gwen can counter because of her unique powers. Or, have the Plumbers from Ben 10's world come to the Fairy Tail world to investigate some strange energy readings that turn out to be related to a powerful magic item in Fairy Tail.
Another idea is to write a story about Ben's descendants in the future. How they use the Omnitrix or a new device based on the same technology. They might face new enemies and have different adventures.
One idea could be to focus on a Celestial Dragon that has been banished from its own realm and ends up in the Fairy Tail world. It could form an unlikely bond with a Fairy Tail member and learn about friendship and loyalty. Another idea is to explore the origin of the Celestial Dragons in the context of the Fairy Tail universe and how they came to be associated with the Celestial Spirit magic. Maybe a young Celestial Dragon is being hunted, and the Fairy Tail guild decides to protect it, leading to an epic adventure.
Maybe a fanfic where the Sorting Hat from Harry Potter is brought to Fairy Tail. The guild members could be sorted into Hogwarts houses based on their personalities. Natsu might be a Gryffindor because of his courage, while Lucy could be a Ravenclaw for her intelligence. This could lead to some interesting interactions and competitions within the guild as they explore the different aspects of the Hogwarts houses.
One idea could be a fanfic where the Dragon Slayers have to team up to face a new, powerful dark dragon that is immune to their normal attacks. They would have to discover new powers within themselves.