A new villain created specifically for the fanfic could be a hybrid of alien and magical powers. This villain could have the ability to absorb the powers of the aliens Ben can transform into and also counter Star's magic. They could be seeking to create a new universe where they are the supreme ruler, and Ben and Star are the only ones standing in their way. They would constantly be evolving and adapting to Ben and Star's attacks, making them a very challenging adversary.
Ben and Star would be the most interesting combination. Ben with his ability to transform into different aliens and Star with her magic wand. They could combine their powers in unique ways. For example, Ben could transform into an alien that can fly and Star could use her magic to enhance its speed.
One idea could be that Ben and Star team up to stop an interdimensional villain who is stealing magical and alien powers. They could combine the Omnitrix's abilities with Star's magic.
Star is definitely one of the most popular. Since she's the main character in the show, a lot of fanfics revolve around her adventures and her growth. Marco is also quite popular as he has a great dynamic with Star. Then there's Tom, Star's ex - boyfriend. His complex relationship with Star makes him a favorite in many fanfics.
In my opinion, Star is the most interesting. She has this amazing ability to adapt to new situations. Whether it's dealing with school on Earth or facing powerful enemies in her own world. Then there's Ludo. He starts as a kind of bumbling villain but as the story progresses, he becomes more complex. And Glossaryck, that little magical being who seems to know everything but doesn't always tell, is really fascinating.
Star would be really interesting. Since she's the main character, she has a lot of secrets and emotions. Her reactions to different truths and dares would be very dynamic, especially when it comes to things related to her magical heritage or her feelings for Marco.
Perhaps a plot where the forces of evil from both universes join together. Ben and Star have to form an alliance with their respective friends and family. They must learn about each other's powers and strategies to defeat this combined evil. Ben might teach Star about his alien tech, and Star could show Ben how to use magic in combat.
I think Toffee is really interesting to write about. His motives were always a bit of a mystery in the show, so in fanfiction, you can really explore what made him tick. Also, Moon Butterfly has a rich backstory and a complex role as a queen. Writers can delve into her decisions and how they affect the story. And Janna, with her quirky personality and her knowledge of the supernatural, can add a lot of fun elements to a fanfic.
Maybe the main villain is a corrupted ninja who once was on the side of good but was seduced by the power of the dark side. He now leads the forces of evil and has a personal vendetta against Naruto Star, seeing him as a threat to his newfound power and plans.
One idea could be exploring an alternate universe where Star and Marco's roles are reversed. Another could be a fanfic that focuses on the backstory of a minor character like Pony Head. Or perhaps a story where the forces of evil have a more sympathetic origin.
Some fanfics focus on the origin stories of the villains in Star vs the Forces of Evil. They might explore what made Toffee so evil or how Ludo became the way he is. By delving into these backstories, the fanfic can add more depth to the existing story. And there are also fanfics that center around the magic items in the show, like the wand, and how they could be used in different and unexpected ways.