It could be about Ben using his alien forms to fight alongside the Straw Hat Pirates in the One Piece world. Maybe he uses Heatblast to help Luffy against a fire - based enemy on a new island they discover.
Well, maybe the Omnitrix gets damaged and its energy somehow mixes with a Devil Fruit. This creates a new situation where Ben has to deal with having powers from both the Omnitrix and the Devil Fruit. And the Straw Hats are intrigued by this new power source and try to help Ben control it while also facing threats from other pirates who want this power for themselves.
Another interesting interaction could be between Gwen (from Ben 10) and Nami (from One Piece). Gwen's intelligence and magical abilities would contrast with Nami's skills in navigation and her love for treasure. They could exchange knowledge, with Gwen teaching Nami some basic magic spells in exchange for Nami teaching Gwen about navigation techniques across the Grand Line.
Ben as Diamondhead and Zoro would be really interesting. Diamondhead's crystal - like body could provide a great defense against attacks, and Zoro's swordsmanship could be enhanced with Diamondhead's sharp edges. They could team up in battles and create some unique combat strategies.
A Naruto - One Piece fanfiction could explore the differences in their power systems. While Naruto has chakra-based abilities, the pirates in One Piece rely on Haki, Devil Fruits, and physical strength. It would be fascinating to see how Naruto would interact with the various characters in the One Piece universe. For example, how would he get along with Zoro, the swordsman? Or how would he deal with the wily Nami when it comes to treasure? There are so many possibilities for an exciting and unique story.
A frequent theme is culture clash. Ben comes from a very different world than the One Piece universe. So, in these fanfictions, there are often parts where Ben is trying to understand the pirate code, the concept of bounties, and the different races in One Piece. Also, adventure is a big theme. The two series are both about going on exciting journeys, and in the fanfictions, they often combine these adventures, like exploring new islands together or facing new and unknown threats.
For Luffy, I think a Pikachu would be a great fit. Just like Luffy, Pikachu is full of energy and has a really positive attitude. They both are also very brave and not afraid to take on opponents much stronger than themselves. For Zoro, a Scizor would be suitable. Scizor is a strong and battle - hardened Pokemon, much like Zoro with his swordsmanship skills. Nami could be paired with a Meowth. Meowth loves shiny things, just like Nami loves money and treasure.
Sakura would fit in as a strong fighter. Her physical strength and combat training from being a ninja would make her a formidable opponent in the pirate - filled world of One Piece. She could hold her own in battles against pirates or other threats.
Well, you could start by looking on fanfiction websites like FanFiction.net or Archive of Our Own. Some popular ones might involve Ben Tennyson somehow getting involved in the One Piece world, like teaming up with Luffy and his crew to fight a new common enemy. But it really depends on what kind of story you're into, whether it's more action - packed, humorous, or something with a bit of mystery.
One interesting concept is a crossover with other popular anime like Naruto. Imagine Luffy and Naruto teaming up, it would be a clash of two very different power systems and personalities. Another concept could be a crossover with a superhero universe. For example, Luffy meeting Superman. Their powers are so different, it would make for an exciting story.