Perhaps they time travel to a time when the Halo rings were first being constructed. They could encounter the Builders of the Halo rings and learn the true purpose behind their creation, which might be different from what they initially thought. They might then be hunted by the Forerunner security systems as they are seen as intruders, and have to fight their way through various challenges while trying not to disrupt the construction process too much.
The story could be that Fireteam Majestic travels to the future. In this future, the Covenant has completely taken over the galaxy. They have to find a way to bring back the knowledge of how to fight the Covenant effectively to their own time. Along the way, they might meet descendants of humans who have been living in hiding and resistance against the Covenant rule. These future humans could have developed new technologies or combat techniques that Fireteam Majestic can use to their advantage.
Maybe the Flood. If they travel to a time when the Flood was at its peak, the Flood could become the main antagonist. The Flood is known for its ability to infect and assimilate all forms of life. Fireteam Majestic would have to fight against the never - ending waves of Flood forms while also trying to find a way to either stop the spread of the Flood in that time or find a way back to their own time before they are all infected.
One possible plot could be that Esme travels back to a time when she was about to make a crucial life decision. For example, she might go back to the moment before she met her significant other. In the past, she could then have the chance to see things from a different perspective. Maybe she realizes that the decision she made back then was actually the best one, or she might find out new things about herself or the people around her that she didn't know before, which could lead to a whole new set of adventures and self - discovery in the past and ultimately change her view of her present life.
Well, a really interesting plot twist in an Eren travels back in time fanfiction could be related to relationships. Say Eren goes back and realizes that his actions in the past have a huge impact on his relationships with his friends. Maybe he accidentally causes Mikasa to not be as attached to him as she was in the original timeline. Or he could find out that Armin was supposed to be the one with the time - traveling ability all along, and his interference messes up Armin's future plans. Also, what if he goes back and discovers that the real enemy wasn't the Titans but some other force that was using the Titans as a distraction, and his time - traveling journey is the only way to expose this truth.
A plot twist could be that Sakura travels back in time and realizes that the world she thought she knew was all a lie. The history books in her time were wrong. She could find that the origin of her magical powers was something very different from what she was taught. Also, she might go back to a time when there was a powerful artifact that was lost in her own time. However, when she tries to retrieve it, she discovers that it has a dark side that she didn't expect. This could lead to a moral dilemma for her, whether to keep it or leave it be.
One possible plot could be Ianto Jones going back in time to prevent a major disaster in Torchwood's history. Maybe he travels back to the time before the Cardiff Rift opened in such a large and dangerous way. He could use his knowledge from the future to stop an alien invasion that was the root cause of the rift's instability.
Perhaps in the 'ichigo travels back in time bleach fanfiction', Ichigo travels back to the time when the Soul Society was in a more fragile state. He could use his knowledge from the future to help strengthen the Soul Society against upcoming threats, like Aizen's plans. He might train the younger versions of the Soul Reapers and form new alliances that would change the future battles.
One possible plot could be Korra going back in time to prevent a great disaster that she has knowledge of in her own time. For example, she might go back to stop the start of a war that had long - lasting negative impacts on the world. She could use her bending skills in new and unexpected ways in the past, as the technology and bending styles of that time might be different.
Perhaps when Bilbo travels back in time, he finds that the Ring has an unexpected origin or connection to other powerful artifacts. This could lead to a whole new set of challenges and adventures as he has to deal with these new revelations while still trying to destroy the Ring. For example, he might find that the Ring is part of a set of items that need to be dealt with together in a specific order.
One possible plot could be that the Master Chief is somehow transported to the Avengers' universe. He teams up with Iron Man to stop a new threat that combines Covenant and Hydra technology. Their combined skills in combat and technology would make for an exciting adventure.
The plot might involve the RWBY team being mistaken for some sort of new Halo soldiers. They could be put through training regimes in the Halo world, which would be really interesting to see as they adapt their own fighting styles to Halo's more military - based combat. Their semblances could also create some chaos or new strategies within the Halo battles. For example, Weiss' glyphs might interact in unexpected ways with Halo's energy shields.