A possible plot is that the male OC discovers a hidden feature in the Omnitrix that gives him the ability to transform into powerful, ancient alien forms. However, this new power attracts the attention of a powerful intergalactic villain who wants to take the Omnitrix for himself. So the OC has to battle this villain while also learning to control his new powers.
One idea could be that the male OC with the Omnitrix accidentally gets transported to an alternate dimension where the Omnitrix functions differently. He has to figure out how to adapt and use it to get back home. Another plot could involve the OC using the Omnitrix to stop an alien invasion that is targeting his hometown specifically. Maybe the aliens are after the Omnitrix's power source.
You could also write a fanfiction where an OC is accidentally bonded with the Omnitrix in a way that it can't be removed. The OC then has to learn to live with it and use its powers for good, but they face challenges as they are not Ben. They might have to deal with people who are suspicious of them having the Omnitrix and also face enemies who are specifically after the device because of its new and unexpected host.
A possible plot is that the self - inserted character has to deal with the Omnitrix malfunctioning. This could lead to unexpected and sometimes comical transformations. They might transform into a really weak alien in the middle of a battle or into an alien that is completely inappropriate for the situation, like a tiny alien when they need to be strong. As they try to fix the Omnitrix, they learn more about its inner workings.
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.
Another could be an OC with the Omnitrix getting stuck in a parallel universe where the rules of the Omnitrix are different. For example, instead of being able to transform into known aliens, the OC has to figure out how to use new and strange alien forms that are native to that universe. This could lead to all sorts of adventures as the OC tries to find a way back home.
One idea could be a story where the Omnitrix is fused with a Pokeball, allowing the protagonist to transform into a Pokemon and use its powers while also being able to capture other Pokemon in a unique way. Another could be a cross - over where characters from Ben 10 (where the Omnitrix is from) end up in the Pokemon world and have to use the Omnitrix to survive and understand the new environment. A third idea is a story where a Pokemon trainer discovers an ancient Omnitrix - like device that gives them the ability to transform into rare and powerful Pokemon forms that no one has ever seen before.
One idea could be that the Omnitrix malfunctions and gives Ben new, unexpected alien forms with powers he has to quickly learn to control. For example, an alien that can manipulate time in small bursts, which would create really cool scenarios where he has to fix things that went wrong in the past few seconds. Another idea is that a new villain is specifically targeting the Omnitrix and Ben has to protect it while also trying to figure out the villain's motives. Maybe this villain has a device that can nullify the Omnitrix's powers temporarily.
One idea could be that Ben discovers a new and powerful alien form in the Omnitrix that has abilities he's never seen before. He has to quickly learn to control it to defeat a new and dangerous villain.
A male oc could be a supernatural being, like a half - vampire or something similar. He comes to Ikebukuro in search of a cure for his condition. Along the way, he befriends some of the main characters, such as Mikado and Kida, and together they uncover secrets about his origin that are linked to the long - forgotten history of the city.
Maybe the Hufflepuff male OC is an exceptional flyer, even though Hufflepuff isn't known for Quidditch. He tries out for the team and faces discrimination from some of the other houses at first. But through his hard work and kind nature, he not only makes the team but also helps to bring unity among the different houses in the Quidditch matches.
Well, a plot could be that the male OC is a time - traveler. He arrives in Camelot during Merlin's time and has knowledge of future events. This causes a stir as Merlin tries to figure out if he can trust this stranger. Maybe the OC is from a future where magic has been fully accepted and he tries to bring that ideology to Camelot. Or perhaps the male OC is an artist who uses magic in his art, and his works start to have strange effects on the people of Camelot, leading Merlin to investigate.