You could write a fanfiction about Ben going to an alternate universe where the Omnitrix works differently. In this universe, the aliens he transforms into have different powers and personalities. Or perhaps a story where Ben's identity as the wielder of the Omnitrix is exposed to the whole world, and he has to deal with the consequences, like becoming a target for every villain and also facing the fear and adoration of the general public.
One interesting idea could be how she would handle the different alien forms. Since Ben 10 has a variety of powerful alien forms, as a girl, she might approach using them in a more empathetic way. For example, when using the Four Arms form, instead of just brute strength, she could use it to protect the innocent more gently.
Another idea is to write a story about Ben's descendants in the future. How they use the Omnitrix or a new device based on the same technology. They might face new enemies and have different adventures.
Well, you could write a 'ben 10 omniverse fanfiction' where Ben loses his Omnitrix and has to rely on his wits and the help of his friends to still fight the bad guys. Or perhaps a story where the Omnitrix starts evolving on its own and gives Ben new and unexpected powers. Also, a plot where an old enemy from the past returns with a new and more dangerous plan.
One idea could be a crossover with the Powerpuff Girls. Ben could team up with the girls to fight a common enemy that is causing chaos in both their universes. Another could be a crossover with the Teen Titans. Ben's alien forms and the Titans' powers would make for some epic battles and cool team - up moments.
Another idea is to have Ben 10 cross over with a futuristic sci - fi setting. His harem could be made up of female alien species he encounters. They could be intrigued by his ability to transform and his heroics, leading to a harem - like situation where they all vie for his attention.
Well, a cool concept for a Ben 10 Transformers crossover fanfiction could be that the Transformers' AllSpark has a strange effect on the Omnitrix. This makes Ben able to transform into new alien - Transformer hybrids. For instance, Four Arms could become a huge hulking form with robotic arms that can transform into different weapons. Also, the Decepticons might try to steal the Omnitrix to use its power to create more powerful soldiers. Meanwhile, the Autobots would team up with Ben to stop them. Maybe there could be a situation where Ben has to use his alien forms' unique abilities in combination with the Autobots' skills to defeat a powerful Decepticon threat.
One idea could be that Ben Tennyson's Omnitrix malfunctions while he's in the Avatar world. So, instead of transforming into his usual aliens, he starts taking on the forms of Avatar creatures. For example, he might turn into a flying bison or a waterbender - like spirit. This would create all kinds of chaos and new adventures as he tries to figure out how to control these new forms and also deal with the political and elemental challenges of the Avatar world.
Another interesting plot could be that Myaax comes from a parallel dimension where she has different powers. In the fanfiction, she accidentally crosses over into Ben's dimension. Ben, being the hero he is, tries to help her get back home while also learning about her unique powers and how they could be useful against some of his usual foes. For example, her powers might be related to energy manipulation, which could be used to enhance the Omnitrix's functions.
One idea could be that Albedo creates a device that can swap powers with Ben, leading to a lot of chaos as they both struggle to adapt to the new powers. Another could be that Albedo and Ben are forced to team up against a new powerful enemy that threatens both their worlds. A third idea might be that Albedo gets stuck in Ben's body somehow and has to pretend to be Ben while dealing with his friends and family.
Well, you could have a fanfiction where the Galactic Empire somehow gets hold of the Omnitrix. Then they try to use it to create super - soldiers. Ben, of course, has to break into their strongholds to get it back. Or perhaps there's a story where Ben is mistaken for a new type of Force - user because his alien transformations seem to give him powers similar to the Force in some ways. He then has to navigate the complex politics of the Star Wars galaxy while trying to clear his name.