Perhaps there's a time - traveling device that was created by the Old Ones but forgotten. Aloy discovers it and uses it to travel to different points in the past and future. In the future, she could see a desolate world where the machines have completely overrun everything. This could give her a new sense of purpose in her own time, to ensure that such a future doesn't come to pass. She could also use the knowledge she gains from these time - traveling adventures to better understand the origins and functions of the machines in her present.
Time travel in Horizon Zero Dawn fanfiction could change the story in many ways. For example, if Aloy could travel back in time, she might be able to prevent the events that led to the fall of the Old Ones. She could stop the Faro Plague from ever happening, which would mean the machines would never have been created as a solution to the plague. This would completely rewrite the world she knows, as the tribes and their cultures have developed around the existence of the machines. It could also mean she might never have become an outcast, as the Nora's beliefs and traditions are also linked to the machines.
One plot idea could be a story where a young hunter discovers a hidden ability to communicate with certain machines. She uses this to form an alliance with a group of friendly machines to protect her village from a horde of corrupted ones. This would be full of action as they fight side by side.
One idea could be exploring Aloy's life before she became the Seeker. Maybe she had a secret friendship with another outcast in her tribe. Another plot could involve a new threat emerging that is related to the old world's technology but in a way Aloy has never seen before. And perhaps a story about Aloy trying to understand the true origin of the machines beyond what she already knows.
A cool Horizon Zero Dawn crossover fanfiction idea is a crossover with the Legend of Zelda. Picture Aloy in the world of Hyrule. She could use her skills to solve some of the puzzles there and maybe help Link in his battles against Ganon. She might be intrigued by the different types of magic and technology in Hyrule compared to her own world. Also, her spear - fighting skills could be a great addition to Link's combat techniques.
One idea could be about how the main character, Aloy, discovers she is pregnant during her adventures and has to balance her mission to save the world with protecting her unborn child. Another could be a story set in a post - apocalyptic tribe where a pregnant woman uses her knowledge from the 'old world' (like Horizon Zero Dawn's advanced technology) to ensure a better future for her baby.
One idea could be a modern-day character traveling back in time to the Hakuoki era and getting involved in the political intrigue. They could use their modern knowledge to try and change the course of events. Another plot could involve a character from the Hakuoki world traveling to the future and being shocked by the technological advancements. Maybe they struggle to adapt to the new world but also find new ways to bring back knowledge to their own time.
Perhaps Rex travels to a time where dinosaurs and humans co - exist. He has to figure out how this is possible and if it's a threat to the future. Or he could go back to the time of the Renaissance and become an apprentice to a famous artist, using his modern perspective to influence the art world in ways that have a ripple effect through time.
One idea could be that Tony Stark travels back in time to prevent the creation of certain dangerous technologies that later cause chaos in his timeline. He could use his genius to subtly alter the course of scientific development.
One idea could be that the Avengers go back in time to stop Thanos from ever getting the Infinity Stones in the first place. They might have to deal with different versions of themselves and the challenges of changing the past without ruining the future.
One idea could be Tsunade traveling back to the time when the Hidden Leaf was first founded. She could use her medical and combat skills to help the early settlers deal with diseases and threats from other villages. Another plot could be her going back to the time of the Third Great Ninja War and changing the fates of some of the fallen ninjas with her knowledge of future events.