It could be a rival wizard from another guild. This wizard might be jealous of Natsu's power and popularity within Fairy Tail. So, they fabricate evidence to get Natsu expelled, aiming to not only remove him as a competitor but also to create chaos within Fairy Tail. They could continue to be a thorn in Natsu's side as he tries to clear his name and get back to the guild.
A powerful dark wizard from outside the guild could be the main antagonist. This dark wizard might be planning to destroy Fairy Tail, and they start by getting rid of one of its strongest members, Natsu. They could be using Natsu's expulsion as a way to divide the guild and make it easier to attack. Natsu, in his expelled state, has to find a way to stop this dark wizard's evil plan and save Fairy Tail.
It could be Zeref. Since he has a complex relationship with Natsu in the original story, he might manipulate things behind the scenes to make the Fairy Tail guild betray Natsu. Maybe he wants to isolate Natsu for his own evil plans.
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 reason could be that he accidentally causes too much destruction during a mission. Maybe he loses control of his magic and wrecks an important place in the magical world, leading the guild to expel him as a form of punishment.
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 dark wizard who has a grudge against Fairy Tail. This wizard might be the one who made Natsu lost in the first place, by using some devious magic to separate him from his guild.
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.
It could be a rogue dragon slayer. Someone who has turned against the ways of dragon slayers and wants to destroy Fairy Tail for some personal vendetta against Natsu or the guild as a whole.
There might be a group of anti - magic fanatics as the main antagonists. They view Natsu's new and extremely powerful magic as a danger to the natural order of the magical world. They would try to eliminate Natsu and any other powerful mages in Fairy Tail who support him, believing that magic should be limited and controlled, not allowed to reach such overpowered levels as Natsu has.
A powerful dark wizard who has a vendetta against Natsu. Maybe he blames Natsu for something that happened in the past, like the destruction of his dark guild by Fairy Tail. So, when Natsu and Happy leave Fairy Tail, he sees it as an opportunity to get revenge.
Another option could be a dark wizard from the past who was once defeated by the founders of Fairy Tail. But Natsu's time - traveling causes some sort of ripple effect that brings this wizard back to power. He then wants to take revenge on the guild and sees Natsu as a symbol of the future that he wants to destroy.