Perhaps the story could center around Ben and the monster girls going on adventures together in their everyday lives. They might encounter other aliens or creatures, and with Ben's powers and the unique abilities of the monster girls, they work as a team to overcome challenges. For example, they could come across a powerful villain who wants to capture the monster girls for some evil purpose, and Ben has to use different alien forms to fight off the villain and protect his friends.
Ben would probably be the protector and the guide in some ways. He'd use his knowledge of the Omnitrix and his experiences with different aliens to help the monster girls. For example, if a monster girl is scared of a certain type of alien, Ben can transform into that alien in a non - threatening way to show her there's nothing to fear.
A fanfiction could focus on a monster girl's birthday celebration. Since they have different cultures, it would be interesting to see how the human characters plan a party that combines both human and monster girl traditions. They might struggle to find the right kind of cake or presents that suit the monster girl's tastes.
In this fanfic, the tickle aspect could serve as a form of communication. Maybe the monster girls use it to show affection or to playfully tease the humans in their everyday life. For instance, a mermaid monster girl could tickle the feet of a human friend when they are sitting by the water, creating a light - hearted moment that showcases their friendship.
First, understand the characters well. Know their personalities, likes and dislikes. For example, if you're writing about Miia, remember her naivete and strong feelings for the protagonist. Second, create interesting scenarios that could happen in their daily lives. It could be something as simple as going to the market but with a monster - girl twist, like Miia scaring the local vendors with her snake - like appearance. Third, use humor and heartwarming moments. Maybe the girls struggle to fit into human society in a funny way, but then they have a tender moment with the protagonist that shows their deep bond.
Some themes are the protection of the monster girls. In fanfics, there could be a threat to their existence, and the human characters have to step up to protect them. Another theme is the exploration of the unique abilities of the monster girls. How they use their abilities in everyday situations, like a slime girl using her shape - shifting ability to help around the house. And the theme of family. Some fanfics explore the idea of the monster girls forming a family unit with the human characters or with each other, which can be really heartwarming.
First, think about the character you want to focus on. It could be a particular monster girl from the series or an original one you create. For example, if you choose a harpy, consider her unique traits like being able to fly. Then, come up with a simple situation, like she gets lost in the city. This gives you a starting point for your story.
The uniqueness of the concept. Combining the idea of living with monster girls and the tickle aspect creates a very different kind of story. It's not something you see often, so it catches people's attention.
One possible plot could be that Ben 10's aliens end up in the Monster High world. For example, Four Arms could get into a friendly strength - testing competition with some of the stronger monsters at Monster High like the werewolves. It would be a clash of different kinds of powers and cultures. Ben, in his human form, might also have to figure out how to fit in and make friends with the unique students at Monster High.
One plot idea could be that a human character discovers a hidden fairy village where monster girls live. He then has to learn their ways and adapt to their unique culture. For example, there might be a centaur - like monster girl who teaches him how to ride through the enchanted forest.
In this fanfiction, it could be that the Necrofriggian babies are the result of some sort of experiment gone wrong. Ben stumbles upon them and realizes that he is the only one who can help. He has to figure out how to reverse the effects of the experiment while also dealing with the fact that the babies are in a very fragile state. He might enlist the help of his friends, but also face challenges from those who don't want the experiment to be reversed.