Natsu was walking through the forest one day when suddenly a group of dark mages emerged from the shadows. They quickly overpowered him with some sort of magic -binding spell. They dragged him away to an old, creepy lair. His friends back at the guild soon realized he was missing and set out on a search mission. Happy flew around frantically looking for any sign of his friend. Lucy, Gray and Erza teamed up and followed some leads they found in the forest. They were determined to rescue Natsu no matter what it took.
One possible motive could be to gain power. Natsu is a Dragon Slayer, and his magic is very powerful. The kidnappers might want to extract his magic or use him as a weapon. Another motive could be for revenge. Maybe Natsu has unknowingly wronged someone in the past, and they are out for vengeance. Or perhaps they are working for a greater evil, and Natsu is seen as a threat to their plans, so they want to get rid of him by kidnapping him.
It could be Zeref. Since Zeref has an interest in powerful magic users and Natsu is a strong dragon slayer, Zeref might kidnap Natsu to study his magic or use him in some dark plan related to his quest for immortality or power.
In many fanfictions where Gil gets kidnapped, it could be for various reasons. Maybe it's by a rival group who wants to use him as a bargaining chip against his friends. For example, in some stories, Uma's crew might be targeted because they are seen as a threat in the Isle of the Lost. Gil, being a part of that crew, gets kidnapped. Writers often use this plot device to create drama and show the loyalty and courage of the other characters as they try to rescue him.
He could marry an original character created specifically for the fanfiction. This character could be a powerful mage from a far - off place who crosses paths with Natsu during one of his adventures.
One of the senior and more experienced members of the guild. They have seen a lot and know how things should be done in the guild. If Natsu acts in a way that goes against their long - established ways, they would not hesitate to berate him.
Gray would be extremely angry. He would immediately start looking for clues as to where Natsu was taken. His ice - make magic would be on full display as he might freeze things in frustration while trying to figure out the situation.
One possible plot could be that the kidnappers are from a rival guild who want to use Natsu as leverage against Fairy Tail. Gray could then embark on a solo mission to rescue Natsu, facing various traps and enemies along the way. He might have to use his ice - make magic in creative ways to overcome obstacles.
It could be Gray. Although they are friends most of the time, in a fanfiction, there could be a plot twist where Gray's envy of Natsu's abilities gets the better of him, and he betrays Natsu. Maybe Gray thinks that by getting rid of Natsu, he can be the top mage in their guild.