The male OC could have a different origin story for getting the Omnitrix. Instead of finding it by chance, he might have been given it by an ancient alien prophet who saw in him the potential to bring peace to the universe. This would set him apart from Ben's story and give him a unique perspective and set of responsibilities in the fanfiction.
His Omnitrix could have a special feature. Maybe it can combine two alien forms into one super - form. This would make his abilities stand out from the regular Omnitrix in the Ben 10 series. He could use this special ability to defeat enemies that Ben couldn't handle with just a single alien form.
He could have a unique personality trait, like being extremely shy but when he transforms using the Omnitrix, he becomes super confident. For example, if he transforms into a big, strong alien, his normal timid self disappears and he becomes a fearless leader in battle.
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.
To create a unique male OC, consider his appearance. He could have a unique scar or birthmark that has a special meaning related to the Omnitrix. Also, think about his skills and knowledge. He might be a genius when it comes to technology because he has spent so much time trying to understand the Omnitrix. His relationships are important too. He could have a mentor figure who is an old alien that he met during one of his early transformations.
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.
Develop a really unique OC. Give them a distinct personality, backstory, and motivation. For example, an OC who is an alien - human hybrid with a complex family history related to different species in the Ben 10 universe. This will make readers more interested in the character's journey with the Omnitrix.
Well, Ben is obviously one of the main characters as the story is centered around him and the Omnitrix. Gwen might also be a main character as she often plays an important role in Ben's adventures, providing support or using her own magical abilities to help out. Grandpa Max could be another main character, offering his wisdom and experience to guide Ben through various situations.
Well, obviously Ben Tennyson is a very popular character. He is the main character who wields the Omnitrix after all. His different alien forms like Four Arms or Heatblast are also quite prominent in the fanfiction world.
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 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 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 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.