One popular idea could be a crossover where Ben 10's aliens interact with Adventure Time's characters in Ooo. For example, Four Arms could have an arm - wrestling match with Finn. Another idea might be a story where the Omnitrix malfunctions and sends Ben into the Adventure Time universe, and he has to find a way to get back while dealing with all the strange things there. Also, a fanfiction could explore what would happen if Jake the Dog got the ability to transform like Ben using the Omnitrix.
One idea could be exploring an alternate universe where Ben has different alien forms with unique abilities not seen in the original series. Another could be a story focused on Ben's relationships with his friends and family in a more in - depth way, like how they react to his alien - changing powers. A third idea might be a time - travel story where Ben goes back in time and has to use his powers to fix something in the past.
One idea for a Ben 10 Ben and Young Gwen time travel fanfiction could be that they travel back to the time when the Omnitrix was first created. They could witness the origin of the device and perhaps even meet the aliens who had a hand in its creation. This could lead to them learning secrets about the Omnitrix that they didn't know before, which could help them in their present - day battles.
In a fanfiction crossover, they could end up in each other's universes by accident. Ben finds himself in a world full of EVOs and has to rely on Rex to get back home. Meanwhile, Rex is intrigued by the Omnitrix and tries to figure out how it works. They face many challenges together, like dealing with Rex's old enemies who see Ben as a new target.
Well, there's this fanfiction called 'The Enchanted Forest Mystery'. In it, Finn and Jake have to solve a mystery within an enchanted forest that has some really strange and magical elements. Then there's 'Marceline's Melody'. It's about Marceline creating a new melody that has some unforeseen consequences in the Land of Ooo. Also, 'Ice King's Redemption' is quite popular where the Ice King tries to become a better person in his own wacky way.
One popular idea could be Ben 10's adventures in different dimensions. He could encounter new alien species and have to use his Omnitrix in creative ways to solve problems. Another could be team - ups with other heroes from different shows, like he teams up with Danny Phantom to fight a common enemy. A third idea might be exploring Ben's life as a normal kid when he's not using the Omnitrix, like dealing with school and family issues.
One interesting idea could be the transformation of Finn into different magical creatures. For example, he could turn into a fire elemental, which would give him the ability to control and generate fire. This would completely change his fighting style and how he interacts with the Land of Ooo.
One idea could be a crossover with Steven Universe. Finn and Steven could team up to save both their worlds from a common threat. They both have a strong sense of justice and adventure, so it would be exciting to see them interact. Another could be a crossover with Gravity Falls. Dipper and Finn could go on a mystery - solving adventure together. The different magical elements of both shows would blend well.
One idea could be Ben traveling back in time to meet his younger self and having to hide his identity while trying to prevent a future villain from getting a powerful weapon in the past. Another could be Ben traveling to different time periods in the Ben 10 universe to gather artifacts that are needed to stop an all - powerful time - traveling enemy in the present. Or he could accidentally end up in a time where the Omnitrix doesn't exist yet and has to find a way back while dealing with the different power dynamics of that era.
One popular idea is crossovers with other popular animated shows like Teen Titans. It's exciting to see the Ben 10 characters interact with the Titans. Another idea could be a crossover with the Powerpuff Girls, imagining how the different powers and personalities would clash or cooperate. Also, crossovers with the Justice League can be really cool, as Ben 10's alien forms could bring a new dynamic to the superhero team.
One popular storyline is where Naruto and Ben 10 characters are brought together in a parallel universe for a quest. During this adventure, a romantic relationship might develop between a character from Naruto, like Sakura, and one from Ben 10, perhaps Ben himself. They have to overcome various obstacles together, both from their different worlds and from the antagonists in the new universe.