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.
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.
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.
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 possibility is that the story could explore how Gwen's personality changes when she has the Omnitrix. She might become more confident or more responsible. Also, there could be new villains who are attracted to the Omnitrix now that Gwen has it. She would then have to face these threats and protect the Omnitrix from falling into the wrong hands.
In this fanfic, the relationships among the Omnitrix aliens - as - humans would be full of surprises. The competitive relationships between some aliens might turn into friendly rivalries. Take Diamondhead and Four Arms, for instance. As humans, they might compete in sports rather than in battles. Their relationships with humans who don't know their true identities would also be interesting. They would have to constantly lie to maintain their cover, which could lead to feelings of guilt and isolation. But at the same time, they could form deep and meaningful relationships with those who accept them for who they are, despite their secrets. This would create a web of relationships that are both complicated and fascinating to explore.
The story could start with Gwen unexpectedly getting the Omnitrix. Then, she might start exploring the different alien forms she can transform into. She'd likely test out their powers in various situations, gradually getting a handle on how to use them effectively.
In the 'Ben 10' series, Ben Tennyson is the main character. He has a device called the Omnitrix which allows him to transform into various alien forms. Gwen Tennyson is Ben's cousin. She also has magical abilities and often teams up with Ben in their adventures.