Another option could be a corrupted version of one of Ben's alien forms. Maybe the Omnitrix gets affected by Dracula's dark magic and one of Ben's aliens becomes evil and starts working for Dracula. This would add an interesting layer of conflict as Ben would have to face an enemy that was once a part of him and also deal with Dracula's overall plan.
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.
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.
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.
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.
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.
One possible fanfic could be one where Ben Tennyson has to face Dracula in a battle that spans different dimensions. Maybe in this fanfic, Ben uses his alien forms in creative ways to combat Dracula's vampiric powers.
Maybe it's a human villain who made a deal with the aliens during the space war. Now that Ben is back, this human villain sees him as a threat to his new - found power and plans to eliminate him. The villain could be a power - hungry scientist or a military leader gone bad.
Obviously, Ben Tennyson and Looma are the main characters. Ben with his Omnitrix which gives him access to a variety of alien forms. And Looma, the strong - willed and powerful Tetramand.