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.
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.
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.
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, 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.
In some fanfictions, there might be female versions of some of the alien species that are popular in the Ben 10 universe. These female alien characters with their own Omnitrix - like devices could be popular. For example, a female Heatblast or a female Four Arms. Their unique abilities and personalities would attract readers, and the stories could explore how they use their powers in different situations compared to their male counterparts.
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.
An old enemy of the Omnitrix's creator could be a great main antagonist. Maybe this enemy was locked away for centuries and now has escaped and blames the Omnitrix for his imprisonment. He would be extremely powerful and have knowledge of the Omnitrix's weaknesses that Ben doesn't even know yet.