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.
There could be a new type of alien - human hybrid villain. Created during the chaos of the space war, this hybrid has a grudge against Ben. It blames Ben for the destruction it has endured and is now on a revenge spree. It has the intelligence of a human and the strength and abilities of an alien, making it a formidable foe for Ben.
Perhaps in the fanfiction, Ben returns but he's a changed person. He has seen the horrors of the space war and now he struggles to adapt to his old life. He might also have some new allies from the space war who come with him to Earth.
Well, a Ben return from war fanfiction can be really interesting. It might explore his post - war trauma, how he readjusts to civilian life. Maybe he has trouble sleeping or is jumpy at loud noises. The fanfiction could also show his relationships with family and friends changing. They might not fully understand what he's been through.
It could be a rogue ninja who took advantage of the chaos during the war. This ninja might have been gathering power in the shadows and now sees Naruto as a threat to his plans.
One possible plot could be Ben trying to reconnect with his old life but finding it difficult. His friends have moved on in different ways and he feels like an outsider. Another plot could be him having flashbacks during normal activities, like when he's at a crowded market or a noisy party.
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.
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.
An evil alien warlord could be the main villain. This warlord could be bent on conquering Earth and other planets in the universe. He might see the Miraculouses as a threat and try to destroy Ladybug and Chat Noir first.
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.
It could be an ancient evil that has been awakened by the chaos of the two universes colliding. This entity is powerful and has the ability to manipulate the very fabric of the Gears of War and Ben Carmine's world. It wants to destroy both worlds and reshape them in its own dark image, and Ben Carmine and the Gears have to figure out a way to stop it before it's too late.