Another aspect could be her fashion sense when transforming. Instead of the typical boyish look of the alien forms, she could add some girly touches. Like when in Heatblast form, she could have a flaming tiara or something similar that shows her femininity while still being a powerful alien.
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.
In the fanfiction, perhaps her relationships with the other characters would be different. She might have a closer bond with Gwen as they could share more 'girl - talk' kind of moments. And with Grandpa Max, he might be more protective in a different way compared to when Ben was a boy.
One idea could be that the Lucky Girl discovers a new alien form that Ben doesn't know about yet. She has to figure out how to use its powers to save the day. Maybe it has the power to manipulate time in a small area, and she uses it to stop a villain from stealing an important artifact.
One plot idea could be that Ben turns into a girl and has to deal with the different social expectations. For example, his friends might treat him/her differently at first. Another could be that the Omnitrix malfunctions further and only allows female forms, so Ben has to learn to master these new forms in battle. Maybe there could also be a story where Ben as a girl gets caught up in a situation where only a girl can save the day, like infiltrating an all - female villain group.
One idea could be that as a girl, Ben has to face different social challenges while still using the Omnitrix to save the day. For example, some might underestimate her because of her gender, but she proves them wrong with her heroic deeds.
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.
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.
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.
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.
One interesting 'ben 10 girl fanfiction' could be about a girl who discovers she has powers similar to the aliens in Ben 10. She might start her own adventure, fighting bad guys and trying to figure out how to control her newfound abilities.
A cool fanfiction concept could be a crossover tournament. Ben 10 brings some of his alien allies, and the Equestria Girls bring their friends from Canterlot High. They compete in a series of challenges that test both strength and magic. Each team has to learn from the other's abilities and work together at times. This could lead to some interesting friendships and rivalries between the characters from the two different universes.
One idea could be a crossover where the Miraculous heroes and Ben 10 team up to fight a common enemy. Maybe an alien villain that is immune to the powers of the Miraculous but Ben's alien forms can counter. Another could be a situation where Ladybug's Lucky Charm combines with one of Ben's alien gadgets to create a super - weapon. And a third could be a story where Ben gets akumatized and the Miraculous heroes have to figure out how to save him without using brute force as his alien forms are very powerful.