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 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.
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.
Maybe the male OC is from a different dimension where the Omnitrix works in a completely different way. He accidentally gets transported to the Ben 10 universe. His Omnitrix has some unique alien forms that are not known in this universe. He teams up with Ben and Gwen to fight a new common enemy that is attracted to the strange energy his Omnitrix emits. Along the way, he learns about the values of friendship and courage from them.
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.
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.
In some persona 4 oc fanfiction, an OC might be a detective who is secretly investigating the strange happenings in Inaba. This OC could have their own set of skills and resources, and as they dig deeper, they discover connections between the real world and the TV world that the main characters haven't noticed yet. Their investigations could also put them in danger and lead to some intense action scenes.
Another could be a love story within the context of the royals. An original character might fall in love with a royal heir. There could be many obstacles such as differences in social status and the expectations of the royal family. But their love overcomes all these difficulties in the end.
One interesting storyline could be about an original character (OC) who is a half - elf with unique magic abilities trying to find their place in the war - torn world of Warcraft. They might struggle between the two sides of their heritage.
A really engaging storyline might be the OC having to train a young rookie. They take this newbie under their wing, but the rookie has some bad habits that the OC has to break. It's a story of mentorship, growth, and the clash of different generations in the WWE world.
One interesting storyline could be an OC (original character) who is a spy behind enemy lines. They have to use their wits and unique skills to gather intelligence without getting caught.