Perhaps Nico's own insecurities could act as an antagonist. His future self might be so different and successful that Nico starts to feel inadequate and his self - doubt tries to prevent him from fully interacting with his future self and learning from the encounter. Another idea is that there could be a group or organization in the story that has rules against time - traveling encounters like this, and they try to stop Nico from having this meeting.
There's a chance that the future self could be the antagonist in a sense. What if the future self has become corrupted or has ulterior motives? He might try to manipulate Nico into doing things that are not in Nico's best interest. Or, if the future self is from a dystopian future, he could be trying to change the past in a way that would have negative consequences for the present, and Nico has to figure out how to deal with this while still trying to understand his own future.
Well, perhaps his future self shows Nico a different path he could take in life. For example, if Nico is currently on a path that leads to loneliness in the future, his future self might encourage him to be more open and make more friends. This could lead to a whole new set of adventures and character development for Nico. It could also be that his future self has achieved great things, and gives Nico the motivation and guidance to strive for similar goals.
Perhaps there is no traditional antagonist. Instead, the conflict could be within Percy himself. His future self might have some dark secrets or regrets that Percy in the present doesn't want to face. So, in a sense, Percy's own denial or fear could be the 'antagonist' in this fanfiction.
Maybe a dark version of Neji himself could be the antagonist. This could be a Neji from a parallel world where he succumbed to hatred and envy. He might try to destroy the 'good' Neji and take over his life, believing that his way is the right way and that the other Neji is a weak version.
Perhaps Neji's own insecurities could be the main antagonist in a sense. When he meets his alternate self, he might start to feel inadequate or question his own identity. This internal struggle could be the main obstacle they have to overcome rather than an external enemy.
In this fanfiction, Steven could learn valuable lessons from his future self. For example, he might find out how to better use his powers or how to handle difficult situations with the Crystal Gems. Maybe his future self would warn him about upcoming threats and how to avoid them.
In the 'natsu meets his future self fanfiction', one possibility is that his future self warns him about certain events that are going to happen. For example, about an upcoming battle that is more dangerous than any he has faced before. His future self could also show Natsu new magic techniques that he will develop over time. It could also be a situation where Natsu's future self is not what he expected, perhaps having made some mistakes that Natsu now has the chance to avoid.
Well, perhaps Percy's future self comes back to warn him about something. It could be about an upcoming danger that he needs to avoid. Or maybe his future self has some wisdom to impart, like how to better handle his relationships with his friends or family. This encounter could change Percy's perspective on a lot of things in his current life.
One interesting scenario in 'naruto meets his future self fanfiction' could be that Naruto's future self has a completely different appearance or set of abilities due to some unforeseen events in the future. This could shock the present Naruto and make him more curious about what the future holds. His future self could also share some secrets about the hidden powers within him that he hasn't discovered yet, leading to a whole new journey of self - discovery for the current Naruto.
Well, it might be that Hiccup's future self shows up at a crucial moment in the present. He could warn Hiccup about a big mistake he's about to make, like trusting the wrong person. This would then lead to Hiccup re - evaluating his decisions and taking a different path. Maybe it also involves some revelations about his future family and how his actions in the present will affect them.
It could be a new villain from the future that somehow follows future Gohan back in time. This villain might be an evolved form of an existing enemy, stronger and more cunning. They could be after the secrets that future Gohan holds about the future technology or the power levels of the Z - fighters in the future.