If Harry Potter got the Ultimatrix in fanfiction, it would be a total game - changer. He might start using the alien forms in his battles at Hogwarts. Imagine him turning into an alien that can fly during a Quidditch match, giving Gryffindor an unbeatable advantage. Also, the magical world would be both fascinated and a bit scared of his new powers. Some might see him as a threat, while his friends would support him in exploring the new abilities the Ultimatrix gives him.
His confidence would likely skyrocket. With the new powers from the Ultimatrix, he'd be able to take on challenges more boldly. He might become more independent as he explores the abilities on his own.
The first thing that might happen is chaos among those who know Ben. His family would be worried sick. If the kidnappers are villains who know about the Omnitrix, they could try to force Ben to unlock all its secrets. But Ben won't give in easily. He'll fight back using whatever means he can, even if he's restrained.
It could be Vilgax. He's one of Ben's most powerful enemies. Vilgax is always looking for ways to defeat Ben, and he might come up with a new and devious plan to hurt him. Maybe he teams up with other villains to ambush Ben when he's least expecting it.
Gwen could also take care of Ben. As his cousin, she would be worried about him. She might use her magic to try and ease his symptoms or find a cure. She would probably make him some soup and keep him company while he's sick.
One possible scenario could be that Ben gets hurt during a fight with a powerful alien. Maybe he underestimates the enemy's strength and gets severely injured while trying to protect someone. Another could be an accident while testing out a new alien form. The transformation might go wrong and cause him physical harm.
There are also the possibility of some inter - dimensional beings. In the Ben 10 universe, there are many different dimensions. If a being from another dimension comes through with the intention of causing chaos, they could target Ben. They might have powers that are completely foreign to Ben and his allies, making it difficult for him to defend against them. And let's not forget about the possibility of some of Ben's old enemies teaming up. If they work together, they could come up with a plan to really hurt Ben, whether it's physically or by ruining his reputation among the people he protects.
Well, it could be that his friends have to step up and protect the city while he's sick. Gwen and Kevin might have to deal with villains on their own, and they could struggle without Ben's usual power. Also, Ben might be really frustrated because he can't use his powers as he normally does when he's ill.
He would probably become an even more powerful ninja. With the Ultimatrix, he could transform into various alien forms, each with their own unique abilities. This would give him an edge in battles against tough opponents like Madara or Kaguya. He could use the alien powers in combination with his ninja skills, for example, using an alien form's strength along with his Rasengan technique.
One possible scenario could be Ben getting hurt during a battle with a powerful alien. Maybe he underestimates the enemy's strength and gets seriously injured. Another could be that he gets hurt while trying to protect someone, like his grandpa or Gwen. It could also be due to some malfunction of his Omnitrix.