Perhaps a powerful interdimensional being that has the ability to control both ghosts and aliens. This villain would be able to manipulate the powers of Danny's ghost enemies and Ben's alien foes, making it a very difficult opponent for them to face. It would force Danny and Ben to really understand each other's powers and find new ways to fight back.
It could be an evil scientist who has found a way to combine ghost and alien technology. He wants to use this to take over both the human world and the ghost zone.
A Danny Phantom and Ben 10 crossover fanfiction would be full of action and adventure. The two characters have very different power sets. Danny has his unique ghost powers such as flight, invisibility, and ecto - blasts. Ben, on the other hand, can transform into a variety of aliens with different abilities like Four Arms' great strength or XLR8's super speed. In the fanfiction, they could be brought together by a common enemy that is threatening both the human world and the ghost zone. They would have to learn to work together, with Danny using his knowledge of the supernatural world and Ben using his combat experience from different alien forms. Their combined efforts would make for a thrilling story as they face challenges that neither could overcome alone.
It's really hard to say. Danny has some incredibly powerful ghost abilities like intangibility, flight, and his ghostly wail. But Ben has access to so many different alien forms, each with their own unique powers. However, if we consider the fact that Danny has a connection to the Ghost Zone which gives him an almost unlimited source of power in a way, he might be the most powerful. But then again, Ben in his Alien X form has near - omnipotent power. So it really depends on how you define 'powerful' in the context of this crossover.
Perhaps a mad scientist from Danny's world who has somehow gotten access to Ultraman technology and is using it to create chaos. He could be creating mutant creatures that are a threat to both the Ghost Zone and Ultraman's world, and Danny and Ultraman have to stop him before it's too late.
I think Eobard Thawne, the Reverse - Flash, could be really interesting. He could use his speed to try and steal Danny's ghost powers. His devious nature and time - traveling abilities would make him a formidable foe. He might see Danny's powers as a way to enhance his own and become even more powerful. And Danny would have to use his unique ghostly skills, like his ability to phase through objects and fly, to outwit the Reverse - Flash.
Well, Danny Phantom has the ability to phase through objects and fly at high speeds, which is really cool. Ben 10 can transform into so many different aliens, each with their own unique powers. And Kim Possible is super athletic and has all those useful gadgets. However, I think Ben 10 might be the most powerful in a crossover. His Omnitrix gives him access to a wide variety of powers that can be used in different situations. For example, if they were in a fight underwater, he could transform into an alien that is adapted to aquatic environments. But again, it's all about how the story is written and what challenges they face.
A possible main villain could be a dark wizard from Star's world who has somehow gained the ability to control ghosts. He could use this power to try and take over both Danny's world and Star's, seeing the combination of magic and the ghost zone as a means to ultimate power.
Maybe the main villain is an ancient spirit that was once locked away in a dimension between the two worlds. Danny's accidental crossing of dimensions and Castiel's presence somehow break the seal. This spirit wants to destroy both the world of the living (that Danny protects) and heaven and hell (that Castiel is associated with) in order to create its own chaotic realm.
Vilgax could be a great main villain. He has a long - standing rivalry with Ben and his presence would add a lot of tension. He could see Albedo as a useful tool or a threat depending on the situation. Another option could be an original villain created specifically for the crossover. Maybe a powerful alien entity that wants to use Albedo's knowledge and Ben's powers for its own evil purposes. A third choice could be Aggregor, who might be after some new power source that is related to both Albedo and Ben.
It could be Sam and Tucker. Since Danny has become a villain, his former friends would likely try to stop him. They would be the main antagonists from Danny's point of view now. They would use their knowledge of Danny and their own skills to try and bring him back to the good side.
One possible plot could be that Danny and Ben meet when a powerful interdimensional villain is causing chaos in both their universes. They initially misunderstand each other and get into a small fight. But then they realize they need to work together. Danny could use his ghost powers to phase through objects and surprise the enemy, while Ben could use his various alien forms to combat the villain's different attacks. For example, using Heatblast to counter a cold - based attack from the villain.