Perhaps a rival guild's ace wizard who is extremely jealous of Natsu's popularity and power. He decides to use the Grand Magic Games as a platform to prove that he is better than Natsu. He might use underhanded tactics to try and defeat Natsu and his team.
Another option for the main antagonist could be an ancient magical being that has been awakened by the energy of the Grand Magic Games. This being sees the games as a threat or an opportunity to gain more power. It starts to target the participants, especially Natsu who has a large amount of magical energy, and tries to stop the games or take over the magical world using the games as a stepping stone.
It could be a powerful dark wizard who has infiltrated the games under a false identity. This wizard might have a vendetta against Natsu or his guild and wants to disrupt the games and cause chaos.
Maybe the main antagonist is an ancient evil that has been awakened by the chaos of the Grand Magic Games. This entity sees Natsu as a threat to its plans and attacks him, resulting in the loss of his arm. The evil being would then continue to try and disrupt the games and cause more harm to the magical world while Natsu and his friends try to stop it.
It could be Orochimaru. He might have found a way to enter the Grand Magic Games world and is using his dark jutsu to control some of the magic users. He wants to use the power of the games to become even more powerful and immortal.
Another option is a rival dragon slayer who sees Natsu's transformation as a threat to his own status. This rival could try to stop Natsu from fully becoming a dragon or try to defeat him to prove his own superiority.
He could win every battle easily.
Natsu is obviously one of the most popular. His fiery personality and powerful dragon - slayer magic make him a central figure in many fanfictions. Gray is also popular, with his cool ice - magic and his constant bickering with Natsu adding some humor. Lucy is often a key character too, as she provides a more human and emotional aspect to the story, especially with her celestial spirit magic.
It could be another powerful mage who is jealous of Natsu's new war god slayer magic and wants to steal it for himself. This mage might have been researching ways to gain such powerful magic for a long time and sees Natsu as an obstacle to his own plans.
Natsu being 'godlike' in the Grand Magic Games in fanfiction could mean he has extraordinary powers. Maybe he has a new form or has mastered a super - powerful spell. It could make for an exciting read as he overcomes his opponents in unexpected ways.
One great aspect of fanfictions about Natsu and Lucy in the Grand Magic Games is the potential for character development. Writers can explore their relationship in the high - stress environment of the Games. For example, they might show how Natsu's protectiveness towards Lucy intensifies during the battles. It also allows for creative interpretations of their magic abilities. Maybe a fanfic could have Lucy discover a new spell to support Natsu in a crucial moment.
One idea could be to focus on Natsu training in secret before the games to surprise everyone with new powers. Another could be a story where Natsu forms an unexpected alliance with a rival team member during the games for a common goal. Maybe he discovers a hidden rule that could change the outcome of the games and has to figure out how to use it to his team's advantage.
There could be a character named Ember, a fire - mage who has the ability to not only control fire but also transform into it briefly. She would be a fierce competitor in the Grand Magic Games, with her unique power making her stand out.