A plot twist could be that during their time travel, they get stuck in a time loop. They keep repeating the same events over and over again, and only by solving a mystery related to their past or future selves can they break free. Also, they could time travel to an alternate dimension instead of just a different time in their own world. In this dimension, the rules of magic are different, and they have to adapt quickly to survive. Maybe Natsu loses his fire - based magic and has to learn a new type of magic to get back home.
One possible plot twist could be that when they time travel, they accidentally change a key event in the past that has a domino effect on their present. For example, they might prevent the formation of a guild that was crucial in their original timeline. Another could be that they find out they are related in some unexpected way through their time - traveling adventures. Maybe Ultear discovers she has a connection to Natsu's family that was lost in history. Or perhaps they time travel to a future where everything they know is in ruins, and they have to figure out how to prevent it in their present.
One plot twist might be that the source of Natsu's time - becoming ability is actually a dark force. He initially thinks it's a great power, but as the story progresses, he discovers that it's slowly corrupting him or has some hidden and dangerous agenda. This could lead to an internal struggle within Natsu as he tries to either control or get rid of this power. Another twist could be that time - based changes Natsu makes end up creating an alternate reality that he has to fix before it's too late.
Well, perhaps in the fanfiction, Natsu and Ultear get married in a grand ceremony attended by all their friends from the guild. After marriage, they decide to go on adventures together. Ultear could use her time - magic to help Natsu in battles, like rewinding time a bit to give him an edge. They also start a family, with Natsu teaching their kids about the importance of friendship and Ultear teaching them magic.
I'm not sure specifically as I haven't read this particular fanfiction. However, it might involve a fictional scenario where Natsu and Ultear's relationship takes a turn towards having a family, with all the emotional and story - driven elements that come with it like how they deal with the pregnancy, how it affects their relationships with others in the story's universe.
Perhaps in this fanfiction, a magical spell goes wrong and creates a baby that has a connection to both Natsu and Ultear. They then have to deal with the consequences. Natsu might try to use his fire magic to keep the baby warm in a rather clumsy way at first. Ultear, on the other hand, uses her time - related magic to create things for the baby like a cozy crib or toys. They face challenges like getting the baby to sleep through the night and protecting it from any villains who might want to use the baby for their own evil purposes.
A possible plot twist could be that one member of Team Hebi, say Karin, is actually from the future and has been manipulating the time - travel events all along to achieve a certain goal, like saving her own clan. This would completely change the team's dynamic and their perception of her.
One possible plot twist could be that Ichigo travels back in time and accidentally changes something that makes him not become a Soul Reaper in the first place. This would completely change his destiny and the future of the Soul Society. Another could be that he travels to a time when an ancient evil was first sealed away, and his presence there somehow weakens the seal, leading to a new threat emerging in his present time.
One possible plot twist could be that when they travel back in time, they accidentally change a small event that has huge consequences in the future. For example, they might prevent a key meeting between two clans, which then alters the political landscape of the ninja world. Another could be that they find out that one of them was actually from that time originally and had their memories wiped in the future. This would completely change their understanding of themselves and their place in the ninja world.
Natsu, being his impulsive self, might initially have a hard time adjusting to more reserved time periods. For example, in a time when magic was more secretive and not used so openly, he might constantly be getting into trouble for showing off his powers. Ultear, on the other hand, who is more calculating, might find it easier to blend in. But in a more chaotic time period, like during a war, Ultear's calm demeanor could be shaken as she has to face more extreme situations. In a peaceful future time, Natsu might become more laid - back as there's no need for constant fighting, while Ultear might feel a bit out of place without her usual problems to solve.
Perhaps characters like Gray or Erza from Natsu's side could be part of the story. They might initially be shocked but then come around to support their friend. On Ultear's side, any of her former comrades or mentors could show up. They could have different reactions, some might be happy for her while others could see it as a threat to their own plans or to Ultear's future in a more negative way.
Another plot twist could be that the past they remember isn't actually real. It was all an illusion created by a powerful mage. When they find out, they have to figure out who the real enemy is and why they were put in this false past. This would completely shift the focus of the fanfiction from their relationship in the 'past' to finding out the truth behind the illusion.