Well, one possibility is that Natsu could join Fairy Tail in a different way than in the original story. Maybe he has some special mission before officially becoming a member. For example, he has to prove his loyalty by helping an elder member with a personal problem.
One possible plot could be that Natsu has been living a different life away from Fairy Tail. Maybe he was raised by another guild unknowingly. When he finally joins Fairy Tail later, he has to prove himself to the existing members who are skeptical of his late arrival. He might have unique magic skills he developed elsewhere that surprise everyone.
Perhaps an old enemy of Fairy Tail who sees Natsu as a new threat. They might target Natsu specifically to prevent him from strengthening the guild. This antagonist could have a long - standing grudge against Fairy Tail and see Natsu as the new hope for the guild, so they try to eliminate him before he can fully join.
One possible plot could be that Natsu and Mirajane join Fairy Tail at different times. Natsu might bring his usual enthusiasm and powerful fire - based magic. He could quickly get into all sorts of adventures, making friends and enemies alike. Mirajane, on the other hand, could use her Take - Over magic to be a great asset in battles. Their joining could lead to new team - ups within the guild, like Natsu and Mirajane going on a mission together to stop a dark wizard who is causing chaos in a nearby town. They'd face challenges, but their combined skills would help them overcome the odds.
It could be a powerful wizard who has a grudge against Fairy Tail and sees Natsu as a new threat. This wizard might have been secretly observing Fairy Tail for a long time and when Natsu joins, they decide to strike. Their magic could be something unique like shadow manipulation or time - based spells.
It could be a powerful dark wizard who has a grudge against Fairy Tail. This wizard might see Natsu and Mirajane's joining as an opportunity to strike at the guild when it's in a state of flux. He could be someone who was once defeated by Fairy Tail in the past and has spent years building up his power for revenge.
Most of the Fairy Tail members would not take it well. Happy, Natsu's closest friend, would be extremely confused and sad. He'd follow Natsu around, trying to get him to come back to the guild. Makarov, the guild master, would see it as a huge threat to the guild's safety. He might send some of the stronger members to keep an eye on Natsu and Zeref. And the younger members like Wendy would look up to Natsu so much that they'd be very disappointed and scared for him, thinking that he's making a big mistake by joining Zeref.
Yes, in a lot of Fairy Tail fanfiction, Natsu can be overpowered. Writers often like to amp up his powers for more exciting storylines. For example, they might give him new fire - based abilities or make his existing ones even stronger to defeat more powerful foes.
In fanfiction, Natsu's age can vary greatly. Some authors might keep him at his canonical age from the original Fairy Tail series, which is around 17 - 18 years old at the start. However, in alternate universe fanfictions, he could be older or younger depending on the plot. For example, in a time - travel based fanfic, he could be much older if the story takes place far into the future or much younger if it goes back to his childhood days.
If Kakashi were to join Fairy Tail, it would be an interesting mix. Kakashi is a highly skilled ninja from the Naruto universe, known for his Sharingan and diverse jutsu. In Fairy Tail, he'd likely bring in a whole new set of combat techniques. His calm and collected demeanor would contrast with some of the more boisterous members of Fairy Tail. He might also form interesting relationships. For example, he could bond with Erza over their shared combat experience and sense of justice. And his intelligence could be used to help plan strategies for the guild's battles.