Gwen could be a great addition. She could be the voice of reason when Ben and Vilgax are at odds or she could have her own powers play a role in the story. Grandpa Max is also a good choice. His wisdom and experience could add depth to the fanfiction, especially when dealing with Vilgax. Another character could be Kevin. His ability to absorb energy could create some interesting interactions with Vilgax and also provide a different kind of friendship or rivalry dynamic with Ben.
Power struggle is a common theme. Vilgax is always after more power and often clashes with Ben or other powerful beings in the fanfics. Another theme is revenge. Since Vilgax has been defeated by Ben many times, he often seeks revenge in these stories.
One popular 'Ben 10 Vilgax fanfic' could be 'Vilgax's Redemption'. It shows a different side of Vilgax where he tries to change his ways. Another one might be 'Vilgax vs. the New Aliens' which has some great action scenes with Vilgax facing off against newly introduced aliens in the Ben 10 universe.
Well, Ben is obviously one of the main characters as the story is centered around him and the Omnitrix. Gwen might also be a main character as she often plays an important role in Ben's adventures, providing support or using her own magical abilities to help out. Grandpa Max could be another main character, offering his wisdom and experience to guide Ben through various situations.
Well, a plot could be that Vilgax gets amnesia and Ben has to decide whether to take advantage of this or help him. This could lead to a whole new dynamic between them as Ben tries to figure out what to do. Also, a fanfiction could explore what would happen if Vilgax became a mentor to Ben in a strange, alternate universe. They could train together and Vilgax could pass on some of his knowledge, but also try to manipulate Ben for his own ends. Or, perhaps a story where Vilgax and Ben are both stranded on a strange planet and have to work together to survive, all while dealing with their mutual distrust.
One idea could be exploring a new alien form that wasn't in the original series. Maybe an alien with the power to control time in a unique way. Another idea is to have Ben go on a mission in a completely different galaxy, facing new challenges and enemies.
Obviously, Ben is a must - include character as he is the main user of the Omnitrix. Gwen can also be a great addition as she has her own powers and can provide support or be a source of conflict. Grandpa Max is another good character, with his experience and knowledge.
Well, obviously Ben Tennyson is a very popular character. He is the main character who wields the Omnitrix after all. His different alien forms like Four Arms or Heatblast are also quite prominent in the fanfiction world.
One interesting storyline could be about Ben exploring new alien forms that were previously unknown in the official series. Maybe he discovers a form that has unique powers that he has to learn to control quickly to face a new and powerful enemy.
In some fanfictions, there might be female versions of some of the alien species that are popular in the Ben 10 universe. These female alien characters with their own Omnitrix - like devices could be popular. For example, a female Heatblast or a female Four Arms. Their unique abilities and personalities would attract readers, and the stories could explore how they use their powers in different situations compared to their male counterparts.