A new alien character could be created as the main villain. Maybe an alien scientist who was exiled from his own planet because of his unethical experiments. He discovers the existence of the Nemetrix on Earth and wants to use it to take revenge on his home planet. He starts by studying Ben and the Nemetrix, and then he devises a plan to steal it and use it to build a doomsday device that could destroy his former planet and potentially Earth as well.
Albedo could be a great main villain. Since he's always been jealous of Ben and his Omnitrix, if he got his hands on the Nemetrix, he could try to use it to create chaos. He might modify the Nemetrix to turn Ben's alien forms against him or use it to enhance his own powers in a twisted way.
Vilgax would also make a good main villain for this fanfic. He's always after powerful alien technology, and the Nemetrix would be no exception. He could see it as a tool to finally defeat Ben once and for all. He might send his minions to steal the Nemetrix from Ben and then use it to create an army of super - powerful predators.
One option could be an alien scientist who is obsessed with the power of the Nemetrix. He wants to study it and use it to create his own super - weapons. He might be from a race that has been at odds with the creators of the Nemetrix for centuries.
Obviously, Dracula himself could be the main villain. He has all the classic powers like turning into a bat, super strength, and mind control. He could use these powers to try and take over the world or at least the area where Ben is currently located.
One idea could be that Ben discovers a new alien form related to the Nemetrix that has powers he's never seen before. This form could have the ability to control time on a small scale, like freezing enemies for a few seconds. Another plot could involve the Nemetrix being corrupted and causing Ben's alien forms to go haywire. He has to find a way to fix it while dealing with villains who want to use the corrupted Nemetrix for their own evil plans.
One idea could be exploring the origin of the Nemetrix in more depth. Maybe there was an ancient alien race that created it for a specific purpose, like hunting down the most dangerous predators in the galaxy. Another plot could involve a character getting accidentally bonded with the Nemetrix and having to deal with its unpredictable powers.
Ben Tennyson is obviously one of the most popular. Since he's the main character of the Ben 10 series, he's at the center of most fanfictions involving the Omnitrix and the Nemetrix. Fans love to write about his adventures and how he overcomes challenges.
An evil alien warlord from a far - off galaxy could be a great main villain. He could have an army of powerful alien creatures at his command. He might be after the Omnitrix, Gwen's magic book, and Kevin's Osmosian powers to become all - powerful. His forces could be causing chaos across the universe, and it's up to Ben, Gwen, and Kevin to stop him.
The Highbreed could also serve as the main villain. They are a powerful and xenophobic race in the Ben 10 universe. In a crossover, they might view the Justice League's world as a place to expand their territory. They could use their advanced technology and large numbers to attack. Ben would have to convince the Justice League that the Highbreed are a serious threat and together they would need to find a way to stop the Highbreed's invasion, perhaps by using Ben's alien forms like Upgrade to hack into their technology and the Justice League's strategic planning.
Vilgax could be a great antagonist. He could hack into the gaming systems to try and defeat Ben in a virtual world where the rules are different. He might use his advanced technology to create digital versions of his minions to attack Ben.
It could be a new alien species that Ben has never encountered before. This species could be territorial and see Ben as a threat to their planet where he's stranded. They constantly try to drive him away or capture him, which makes it difficult for Ben to find a way to leave.
Maybe a malevolent scientist who has a grudge against female heroes. He believes that only men should have such powers and sets out to take the Omnitrix from Ben and use it for his own evil purposes. He underestimates Ben because she is a girl but soon realizes his mistake.