Who could be the main antagonist in a 'Ben 10 Stuck as Alien Fanfic'?

Perhaps a half - alien, half - vampire hybrid. This character was shunned by both species and blames Ben for making him feel even more like an outcast. He uses his unique powers to create chaos and tries to turn Ben into a vampire - alien like himself.

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.

It could be Orochimaru. Orochimaru is always interested in strange experiments and new forms of power. He might see Naruto in his female form as an interesting specimen to study or use for his own evil purposes.

Another possibility is that an alien race that Ben has previously encountered and thought was friendly, now turns against him/her because of the change in gender. They might have some strange beliefs or customs that make them view Ben's new form as a threat or an abomination, and thus they become the main antagonists in the fanfic.

Vilgax could be a great main antagonist. He has a long - standing feud with Ben and his appearance would immediately up the stakes. He could also manipulate Kevin into turning against Ben for a while.

Perhaps the main antagonist could be a former ally of Ben who has turned bad. After dawn, this character reveals their true intentions and starts causing trouble for Ben. They might have some sort of grudge against Ben or be power - hungry and see the Omnitrix as a means to gain ultimate power. They could use their knowledge of Ben's abilities and weaknesses to their advantage.

It could be an evil Highbreed scientist. This scientist might be obsessed with creating a new breed of Highbreeds that combines the power of the Omnitrix with their own genetic superiority. He sees Ben as the perfect test subject for his experiments and will stop at nothing to achieve his goals.