Maybe a story where Ben teams up with some of his alien enemies against a greater threat in space. They could be in a situation where they have to fight a powerful cosmic entity that wants to destroy all known planets. Ben would have to convince his former foes that they need to work together by showing them the magnitude of the danger.
A plot could involve Ben getting stranded in a strange part of the universe where the laws of physics are different. His Omnitrix starts acting up and he has to rely on his wits and the basic powers of his aliens to survive. He discovers new forms of his aliens that are adapted to this strange environment as he battles various local threats.
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.
One idea could be Ben getting stranded on an alien planet full of strange and dangerous creatures. He has to use his alien forms in creative ways to survive and find a way back home.
One idea could be Ben using his alien forms in a virtual reality gaming world. He could face off against digital versions of his enemies and have to use his unique alien abilities to progress through levels.
One idea could be exploring the origin of the Nemetrix in more depth. Maybe there was an ancient alien race that created it for a specific purpose, like hunting down the most dangerous predators in the galaxy. Another plot could involve a character getting accidentally bonded with the Nemetrix and having to deal with its unpredictable powers.
One idea could be Looma challenging Ben to a series of alien - based contests in her home planet. This would show off different Ben 10 aliens' abilities in a new and exciting setting.
A more character - driven plot could be about Ben trying to help an alien species that is being persecuted in the Omniverse. He uses his different alien forms to understand their culture and problems better. Along the way, he discovers that there are some dark secrets in the Omniverse's politics that are related to this species' situation. This would allow for a deeper exploration of the Omniverse's world - building and Ben's character growth.
Another plot could be that a new villain emerges who has the power to control both human and Myaxx technology. Ben and Myaxx must figure out a way to break this control and stop the villain from taking over the galaxy. They start by gathering intelligence on the villain's lair and formulating a plan that utilizes Ben's shape - shifting abilities and Myaxx's knowledge of her own technology.
A plot could involve a group of young Highbreed who are curious about Earth and its cultures, and they secretly befriend Ben. This would lead to a series of adventures where they have to hide their friendship from the more traditional and hostile Highbreed. They could learn a lot from each other, and Ben could teach them about the value of diversity and cooperation.
One idea could be an OC who has a unique alien form that is a combination of two different Ben 10 aliens. This OC could struggle with controlling its powers at first. Another plot could involve the OC coming from a different dimension where the rules of the Omnitrix are different. They then have to adapt to the Ben 10 universe while also dealing with villains who want to use their unique powers for evil.
One plot idea could be that Ben and Charmcaster have to work together to stop a powerful magical - technological hybrid villain. Ben uses his alien forms and Charmcaster uses her magic, and along the way they learn to respect each other's powers more. Another idea is that Charmcaster gets trapped in one of Ben's alien worlds and Ben has to help her get back home while facing the unique challenges of that world. A third idea could be that they both get sent back in time to a crucial point in the history of their respective powers and they have to figure out how to get back to their own time without changing history too much.