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.
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.
One idea could be exploring a new alien form that wasn't in the original series. Maybe an alien with the power to control time in a unique way. Another idea is to have Ben go on a mission in a completely different galaxy, facing new challenges and enemies.
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.
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 interesting concept could be exploring new alien forms that the Omnitrix can transform into. Maybe there are some hidden or rare aliens that Ben discovers in the fanfiction.
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.
Some fanfictions might explore the idea of a Fem Omnitrix having unique features. For instance, it could have an enhanced ability to transform into female - only alien species. This could lead to new adventures and storylines. Additionally, the fanfiction could focus on how the female character's identity is affected by having the Omnitrix. She might struggle with the power and responsibility in a different way than Ben, which would add depth to the story.
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.