Another storyline could involve Ben facing off against a new villain who is specifically after the Biomnitrix. This villain might have his own set of powers that make him a formidable opponent. Ben would then have to figure out how to use the new aliens in the Biomnitrix to defeat this enemy. Maybe he has to team up with some of his old alien forms in creative ways.
A really cool storyline could be where the Biomnitrix starts malfunctioning and Ben has to go on a journey to find a way to fix it. Along the way, he meets other characters who have knowledge about the Biomnitrix. They might offer him different solutions, some of which could be risky. Ben has to decide which path to take to save the Biomnitrix and continue being a hero.
Gwen Tennyson also often appears in fanfics. She has her own powers and can be a great ally or sometimes a source of conflict for Ben. Her magic abilities can add an interesting dynamic when combined with Ben's Biomnitrix powers.
One interesting storyline could be Ben and Gwen having to face a new, powerful alien threat that they've never encountered before. They might have to combine their unique skills and powers to defeat it.
One interesting storyline could be Ben using his alien forms in a virtual gaming world to defeat a powerful digital virus. The unique combination of his real - world powers and the challenges of the game environment makes it exciting.
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.
Some 'ben 10 omniharem fanfic' might explore how the omniharem affects Ben's growth as a hero. Maybe the relationships make him more empathetic or teach him new ways to use his alien forms in different social situations. In these fanfics, the interactions within the harem could lead to character development not just for Ben but also for the other members of the harem.
Another storyline could involve a love interest between Ben (in one of his alien forms) and a female alien. For example, if Ben turns into Heatblast and meets a female alien who has a natural affinity for fire - like she can control different types of flames. They start off as rivals, competing to see who can control fire better, but then gradually develop feelings for each other as they face challenges together and learn more about each other's cultures.
One interesting storyline could be a female version of Ben having to deal with different alien cultures in the Omniverse in a more diplomatic way than Ben usually does. For example, she might use her charm and intelligence to resolve conflicts between warring alien species on a planet instead of just using brute force with the Omnitrix transformations.
One possible storyline could be Ben and Julie going on an adventure together to a new planet. They encounter strange alien creatures and have to use Ben's alien forms and Julie's martial arts skills to survive. Another could be a story where they face a powerful villain who is threatening their hometown, and they must work together to stop the villain and protect their loved ones.
A really interesting plot could involve a time - traveling aspect. Ben and Julie accidentally travel back in time to when Ben first got the Omnitrix. They have to be careful not to disrupt the timeline while also trying to find a way back to their own time. Along the way, they could meet younger versions of characters they know and have some really fun and emotional interactions. This could also lead to Ben learning some important lessons about his powers from his past self.
Perhaps a story where there is a rivalry between two dance groups at the academy. They compete for the lead in a big performance. Along the way, they learn from each other's strengths and weaknesses, and in the end, they combine their talents to create an amazing show.