A rogue Imperial officer who has his own agenda and sees Starkiller as a threat to his plans. This officer could be secretly building his own army and Starkiller stumbles upon his operation.
A new Sith apprentice, created by Darth Vader in secret. This apprentice is jealous of Starkiller and wants to prove himself by defeating him. He could have unique Sith powers and a vendetta against Starkiller.
The Nightsisters could make great villains. They have their own dark magic and could be after Starkiller for some reason, like wanting to use his power for their own dark rituals.
Starkiller is the central character in this story. He's likely to be a character with unique abilities and a significant role in the Star Wars universe.
One way to integrate Starkiller into the Clone Wars in fanfiction is to make him a time - traveler. He could be sent back in time to the Clone Wars era, and his knowledge of future events could cause all sorts of chaos. He might try to prevent certain tragedies from happening, like the fall of the Jedi Order, which would put him at odds with the Sith who are plotting those very events.
In Star Wars fanfiction, Starkiller could potentially be a rogue agent during the Clone Wars. He might use his unique Force abilities to either interfere with the plans of the Sith or try to find his own path in the chaos of the war. Maybe he teams up with some Jedi who see his potential for good, despite his dark past or training.
Perhaps Starkiller discovers a long - lost Jedi power that was thought to be extinct. He then has to learn how to master it while being hunted by the Empire. This could involve him seeking out ancient Jedi holocrons for guidance.
The main villains could be the Kree. Since they are often at odds with the Guardians of the Galaxy, they might see Star Lord's daughter as a threat or a means to get to Star Lord. They could try to capture her to use her as a bargaining chip.
It could be some new evil organization that combines the worst elements of both the Phineas and Ferb world and the Star Wars universe. For example, a group of evil scientists from Phineas and Ferb's world who have teamed up with the Empire's remnants. They could be planning to create a weapon that would destroy all the fun and creativity in both universes.
Starkiller can be a game - changer in Star Wars crossover fanfiction. He can bring his combat skills and Force knowledge to new settings. For example, if he's in a crossover with the Matrix universe, his ability to sense and manipulate the Force could be compared to Neo's ability to control the Matrix.
You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own. These platforms have a wide variety of Star Wars fanfiction, including ones related to Starkiller in the context of The Clone Wars. Just use the search function and type in relevant keywords.
A Star Wars Starkiller and Padme crossover fanfiction might explore an alternate timeline. Starkiller, with his unique abilities, could interact with Padme in a way that changes the course of the pre - existing Star Wars story. For instance, he could use his skills to help Padme in her diplomatic efforts, or they could team up against a common enemy that was previously unseen in the original plot.
Finn is also a main character. He was a stormtrooper in the First Order. His defection and his knowledge about Starkiller Base are important parts of the story. Another main character is Rey. Although her direct connection to Starkiller Base might not be as straightforward as the others, she is part of the overall fight against the First Order which includes dealing with Starkiller Base.