One interesting idea could be exploring an alternate universe where Ben's Omnitrix has different aliens with unique powers. For example, aliens that can manipulate time in ways not seen in the original series. Another idea is to focus on a story where Ben has to team up with his enemies to face a greater threat. This would create a lot of interesting character dynamics.
One idea could be exploring Ben's new abilities as Ben 110 when dealing with the Highbreed. Maybe he discovers a unique power that can temporarily transform Highbreed into a more peaceful form. Another could be a story where Ben 110 has to team up with a group of Highbreed rebels against the tyrannical Highbreed leaders. It could also involve Ben 110 traveling to the Highbreed homeworld and learning about their culture and how to better co - exist with them.
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 a story where Ben and Gwen have to team up to face a new alien threat that is immune to Ben's usual alien forms. Another could be a fanfic set in an alternate universe where Ben and Gwen have swapped their abilities. Maybe a story about their adventures during a time - traveling incident.
One idea could be exploring a new alien form that wasn't in the original series. Maybe an alien with the power to control time in a unique way. Another idea is to have Ben go on a mission in a completely different galaxy, facing new challenges and enemies.
A 'ben 10 eon fanfic' could focus on Eon's powers. For example, a story where Eon loses his powers temporarily and has to rely on his wits and the help of some unexpected allies. This could show a different side of him, perhaps a more vulnerable side. Or it could be about Eon creating his own army of alternate versions of Ben's aliens, and Ben has to find a way to counter this unique threat.
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.
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.
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.