Perhaps a group of mages from a time when magic was used for more evil purposes. They see Natsu as an intruder and also as a potential source of information about the future's magic. They might try to capture Natsu and use his knowledge to gain more power and spread chaos. Another idea is a time - warped version of Zeref. Since Zeref has a long and complex history, a distorted version of him from the past could be a very menacing antagonist in a Natsu time travel fanfiction.
It could be a dark mage from the past who was sealed away but gets released because of Natsu's time travel. Maybe a mage who was trying to conquer the world back then and now sees Natsu as a threat to his new plans. Another option is a time - traveling villain who is trying to change history for his own evil purposes and Natsu has to stop him.
It could be a dark wizard from the past who has the power to manipulate time. This wizard might see Natsu and Mira as threats to his plans to change the past for his own evil gains. He would try to stop them from interfering with his time - related schemes.
It could be a time - traveling villain from the future who wants to change the past to gain more power in their own time. They might try to stop Naruino from achieving their time - travel goals.
It could be Zeref. If Natsu goes back in time, Zeref might have different plans or be in a more powerful position at that time. He could be the main obstacle for Natsu to overcome, especially if Natsu is trying to change things related to the dark magic that Zeref wields.
It could be a time - traveling version of Zeref. He might be trying to stop Natsu from changing things in the past that could potentially lead to his own downfall in the future.
A new, time - traveling villain could be introduced. This villain could be from a different dimension where time works differently. He comes to Ninjago to steal its time - related artifacts to power his own dimension. His unfamiliar powers and motives would make him an interesting and unpredictable antagonist.
Perhaps an alien race that has mastered time travel long before humans. They see Earth as a target for conquest, and they use time travel to disrupt history and weaken the planet's defenses. Peggy would have to figure out their plans and find a way to stop them. This would introduce an interesting sci - fi element to the fanfiction, with Peggy having to deal with advanced alien technology and culture.
The main antagonist could be a virus that has infected the game's time - travel mechanism. This virus is spreading chaos throughout different time periods in the game world. It's trying to erase certain characters or events from history, and Ralph has to stop it. The virus could take on different forms in different time periods, making it very difficult for Ralph to combat.
It could be an alternate - version of Tony Stark from a different timeline who has gone rogue. This version might have different values due to the differences in his timeline's history and is now causing problems in the main Stark's timeline.
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.
One main theme is self - discovery. When Natsu travels through time, he often has to re - evaluate who he is and what his purpose is. Another theme is the power of friendship. Despite being in different time periods, Natsu's relationships with his guild mates play a crucial role.