Sherlock would likely be enraged. He'd start by retracing John's steps. He'd probably interrogate anyone who might have seen Moriarty or any suspicious activity. He'd also be thinking about Moriarty's endgame. Since Moriarty is his arch - enemy, he knows that this isn't just a random kidnapping. Sherlock would be desperate to save John and would stop at nothing to find him.
Sherlock would be extremely worried. He might go into a state of hyper - focus, using all his skills to find John. He would analyze every little detail, from the place where John was kidnapped to any possible witnesses.
Sherlock might initially seem unperturbed, which could annoy Moriarty. He'd use his sharp mind to analyze Moriarty's motives and actions. He could also try to engage Moriarty in conversation to gain more information. He might say things that would make Moriarty second - guess his own plans. For example, he could point out flaws in Moriarty's previous schemes, making Moriarty angry or distracted. This would give Sherlock an opportunity to either escape or plan his next move.
In a 'Moriarty kidnaps John' fanfiction, it could go many ways. Moriarty is a mastermind. He might have an elaborate plan. He could be using John as a pawn in a bigger game against Sherlock. For example, he may force John to record messages to Sherlock that are full of false information, leading Sherlock on a wild goose chase while Moriarty sets up more traps. John, on the other hand, would likely be trying to find a way to escape or leave some clues for Sherlock.
One possible plot development could be Sherlock using his brilliant deductive skills to outwit Moriarty even while being kidnapped. He might leave behind subtle clues for Watson to find him. For example, he could use Morse code by tapping on the walls if his hands are tied. Another development could be Moriarty trying to break Sherlock's spirit by taunting him with past failures or the safety of his loved ones. Maybe Moriarty has a grand scheme that involves Sherlock in some way, like using him as a hostage to gain access to certain confidential information from the British government.
He might be initially shocked. Sherlock is used to John being more understanding, so this outburst could catch him off - guard. But then he could go into his logical mode and start analyzing why John would say such a thing.
Tony would likely be defiant. He's not the type to cower in the face of danger. He might immediately start looking for weaknesses in Thanos' setup and try to escape on his own terms.
Another reaction could be that Marinette pretends to cooperate with Adrien at first. By doing this, she can buy time and look for an opportunity to escape or get help. She might act as if she is okay with what is happening while secretly planning her next move, like leaving a clue for someone to find her or finding a way to disable Adrien temporarily.
Naruto might initially be shocked and angry. He has always considered the Kyuubi as an enemy within him, and now being kidnapped by it is the worst - case scenario for him. He could try to fight back immediately, using whatever little chakra he can muster without relying on the Kyuubi's power.
She could be terrified at first. After all, Klaus is a very powerful and dangerous vampire. But Elena is also a strong - willed character. So she might quickly gather her courage and start looking for ways to escape.
Kagome would most likely be very angry at first. She's used to being in control of her own life and suddenly being kidnapped by Sesshomaru would be a huge violation. She might try to use her spiritual powers to fight back or at least escape.
At first, Harry would likely be shocked and angry. He's used to being in dangerous situations, but being kidnapped by Fleur, someone he thought was an ally, would be a big betrayal. He'd probably try to fight back immediately, using his magic if he could.