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.
A group of rogue slayers who believe that Natsu's war god slayer magic is too dangerous and needs to be destroyed. They might think that Natsu will lose control of his power and cause chaos in the world. So, they become the main antagonists, constantly trying to hunt Natsu down and take away his magic.
Maybe a dark entity that was once sealed away by the war gods themselves. Now that Natsu has the power of a war god slayer, it sees him as a threat and tries to eliminate him. This entity could be controlling other villains and using them to get to Natsu.
One possible plot could be that Natsu discovers his new power as a war god slayer magic user. He then has to face a series of powerful enemies who are threatening the world. Along the way, he might meet new allies who also have unique magic abilities. As he battles these foes, he gradually masters his war god slayer magic and becomes a more powerful wizard.
A powerful dark dragon slayer who has mastered both shadow and ice magic could be a great antagonist. He could be jealous of Natsu's natural ability with fire and his popularity among the guild, and thus wants to prove his superiority.
Maybe it's an old enemy who survived the war and has grown stronger. They hold a grudge against Natsu and his friends and are now seeking revenge. For example, if there was an enemy that was thought to be defeated before the war but managed to survive and is now back with a vengeance.
Perhaps an ancient evil that has been sealed away for centuries but has now found a way to use the concept of the Death God to break free. This entity was once a powerful ninja who turned to dark magic and now wants revenge on the modern ninja world. It could be a very powerful foe for Naruto as it has knowledge of old and forgotten jutsus.
The dragon god itself could be the main antagonist. Given its power and the threat it poses to the Naruto universe, it would be a natural choice. It could be a being that wants to destroy the ninja world or enslave it for its own purposes.
An ancient dragon that has turned evil due to some dark magic could be the main antagonist. This dragon might see Natsu as a threat because of his unique combination of powers and wants to eliminate him. Natsu would then have to use all his skills and the help of his friends to fight this dragon. The dragon could also have minions that are constantly causing trouble for Natsu and his guild.
It could be an evil ice mage who wants to use the Ice God Slayer's power to freeze the entire world. This mage might have a tragic past that led them to believe that freezing the world is the only way to create peace.
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 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.
Perhaps a forgotten or banished god could be the main antagonist. This god, filled with resentment for being cast out, could return with a vengeance and start causing chaos. He might be powerful enough that the only solution Percy sees is to slay him. This antagonist could bring in a lot of new lore and backstory to the fanfic as Percy uncovers the secrets of this god's past.