It could be an alien bounty hunter. This bounty hunter has been hired to capture Ben in his stuck - as - alien form. The hunter might have some special gadgets or powers that make it difficult for Ben to escape. And he's not interested in Ben's well - being, just in getting the reward for bringing him in.
One idea could be that Ben has to adapt to the daily life of the alien he's stuck as. For example, if he's stuck as a Four Arms, he has to deal with constantly breaking things because of his size and strength. Another could be that he has to find a way to communicate with his friends and family in his alien form, which might be difficult if the alien has a different language or way of expressing itself.
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.
First, focus on the character development. Show how Ben changes mentally and emotionally while stuck in alien form. Second, create interesting conflicts. For example, he could have trouble interacting with his family or friends. Third, describe the alien form in detail. Let the readers really understand what it's like for Ben. For instance, how does it affect his senses? His movements?
There's a fanfic called 'Alien Exile'. In this story, Ben is not only stuck in alien form but also exiled from his home because people are afraid of his new appearance. It's a really emotional read as he tries to find a place where he belongs. Another one is 'Forever Alien'. This one takes a different approach by showing how Ben starts to embrace his alien form over time. He discovers new abilities and a new sense of self. And 'Alienated Ben' is a fanfic that focuses on the social isolation he experiences while stuck in alien form and how he overcomes it.
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.
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.
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.
Loki could also be a main antagonist. He could see Ben as an interesting new player in the universe and try to manipulate him for his own gain. Or he might want to use the Omnitrix to cause chaos across the multiverse.