Loki could also be a main antagonist. He could see Ben as an interesting new player in the universe and try to manipulate him for his own gain. Or he might want to use the Omnitrix to cause chaos across the multiverse.
Another interesting plot could be that some of Ben's alien forms are mistaken for new mutants in the Marvel Universe. He then has to prove that he's not a threat while also helping the X - Men deal with a common enemy that is interested in the genetic makeup of both mutants and Ben's alien forms.
A Ben 10 and Marvel fanfic can stand out if it plays with the concept of time and space. Since Ben has access to different alien forms which could potentially mess with the fabric of space - time, and Marvel has characters like Doctor Strange who are masters of time - related magic. You could have a story where Ben accidentally disrupts the space - time continuum in the Marvel universe and has to team up with the Avengers to fix it. This would not only create a unique plot but also allow for some really interesting interactions between the two universes' characters.
I haven't actually read this specific fanfic, but generally in a 'Ben x Esther' fanfic in the Ben 10 Omniverse setting, it might explore their relationship in different ways. It could involve adventures where they team up, and their characters' growth while dealing with various threats in the Omniverse. Maybe it also delves into how their different personalities interact and complement each other.
One great 'Ben 10 Ben x Gwen Fanfiction' is 'Parallel Bonds'. It explores their relationship in an alternate universe where they have to work together against a new kind of enemy. The story is full of action and also delves into their emotional connection.
One great Ben x Kevin fanfiction could be about their adventures in a parallel dimension. In this story, they might face new aliens and challenges that test their friendship even more. Maybe they find a powerful alien artifact that only works when they combine their unique abilities, leading to a deeper understanding of each other.
In a 'Ben 10 Ben X Young Gwen Time Travel Fanfic', the possibilities are endless. They could travel to the future first and see a dystopian version of their world where the bad guys have won. This spurs them to go back in time to prevent it from happening. Along the way, they might meet their future selves who give them clues or warnings. Their time - traveling could also cause some unforeseen consequences, like creating alternate timelines. For example, if they change one small event in the past, it could lead to a completely different Ben 10 universe in the future. So they have to be very careful with their actions during their time - traveling journey.
One common theme is growth. Both Ben and Gwen grow as individuals and in their relationship. Another theme is friendship, as their bond is often a central part of the stories. Adventure is also a big theme, since they are from the Ben 10 universe and usually go on exciting adventures together.
One idea could be Ben Tennyson using his Omnitrix to team up with Spider - Man. Ben could transform into XLR8 to chase down villains while Spider - Man swings through the city. Another could be having Four Arms help the Hulk in a battle, combining their brute strength. And perhaps Heatblast could join forces with the Human Torch to fight a fire - based villain.
Captain America would be a great addition. His leadership and moral compass could guide Ben in some tough situations. And then there's Wolverine. His healing factor and fierce fighting style would contrast well with Ben's more alien - based combat. They could have some epic battles side by side.