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.
If Ben is kidnapped in this fanfiction, his friends will immediately start looking for him. They'll search every possible place. Maybe they'll get some clues from other aliens who have seen something unusual. Ben, on the other hand, will be trying to figure out the kidnappers' motives. He might even try to befriend one of the kidnappers' minions to get some information and plan his escape.
In a Ben 10 fanfiction where Ben is kidnapped, it's a really tense situation. The kidnappers could be after his unique ability to transform into different aliens. Ben would be in a tough spot. However, he's not alone in spirit. His friends and family's love and determination to find him would be a strong force. Ben might also discover new things about his Omnitrix while in captivity, like a hidden function that could help him break free. And when his rescuers finally come close, he could use his alien forms to create a distraction and make it easier for them to get to him.
Chapter 917, "The Civet Cat Replaces the Crown Prince", is the clearest early answer because it turns Meng Xuezhi's situation into a concrete plot event.
Chapter 599, "The Weather has Cooled Down, the Merchant Should Go Bankrupt", is the clearest early answer because it turns Shang Zhenhe's situation into a concrete plot event.
If you want the clearest chapter reference, go to Chapter 13. Early on, Sensing that something was off with Jiang Yan, Jiang Yan dismissed the...; later, the chapter leans on What on earth do you mean?" Ignoring him once again, Jiang Li shifted her....
Chapter 680, "One Hundred Million has been credited to your Alipay account!", is the clearest early answer because it turns Miss Jiang's situation into a concrete plot event.
Chapter 637, "Does President Shang Know That His Own Family Is Already Bankrupt?", is the clearest early answer because it turns Is Bankrupt's situation into a concrete plot event.
Chapter 635, "Forced Return to the Time-Space Administration", is the clearest early answer because it pushes Zhuola into a new phase or setting. The chapter gives readers a real answer by opening on But Pei Ye was much quicker than she had imagined.
Chapter 701, "Let Bai Lu and Chen'an Terminate the Contract", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Miss Jiang, that move of yours was totally awesome!" Shao Changqing...
Chapter 882, "Can you still not forget your first love?", is the clearest early answer because it turns Lin Jingshan's situation into a concrete plot event. It opens with Seeing the evidence Lin Jingshan presented, Xu Mei's body trembled...
Chapter 581, "Where is everyone? Where have they all gone?!", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event. It opens with Inside the tent, no one felt like sleeping. and closes on Hurry up, or else we'll be left with nothing once those people....
Chapter 736, "When Will You Come to My House for Dinner?", is the clearest early answer because it turns Zhao Lang's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Walking outside the tent, Zhao Lang waved his hand.