A really interesting plot would be if a time - traveling villain from another dimension comes to the HTTYD world and tries to change the past to make dragons evil. Hiccup and his friends then have to travel through different time periods to stop this villain. Or perhaps the characters travel to a time when there were giant, prehistoric dragons that they have to deal with, and they learn new ways of dragon - riding from the ancient Vikings who tamed those behemoths.
One idea could be Hiccup traveling back in time to the era when dragons first arrived in Berk. He could witness how the initial interactions between humans and dragons began and use his modern - day knowledge to try and improve relations even earlier. Another could be Toothless accidentally activating a time - travel device and taking Hiccup to a future where dragons are extinct, making them strive to find a way to prevent such a future.
One idea could be Batman traveling back to the time when Gotham was first founded. He could try to prevent the seeds of corruption from being sown. Another plot could be him traveling to a future Gotham that has been completely overrun by villains and he has to find a way to change that future by altering events in the past. Or perhaps he travels to different time periods to gather knowledge and technology to better fight crime in his own time.
A cool plot for a Pokemon time travel fanfic might be that a Pokemon researcher travels back in time to study ancient Pokemon species that are now extinct. Along the way, they encounter problems like not being able to use modern technology to catch or study the Pokemon. They might have to rely on the knowledge of the people in that era. Also, a trainer could travel through time to meet their ancestors who were also Pokemon trainers and learn about their family's history in the Pokemon world.
One idea could be that Luffy travels back to the time when Gol D. Roger was still alive. He could meet the Pirate King and learn some of his secrets. Maybe Roger gives him a hint about the One Piece that changes Luffy's understanding of his journey.
Maybe a character from the future RWBY world travels back to the time when the Huntsman academies were first founded. They could introduce new fighting techniques or ideas that were developed in their time, which would have a huge impact on how the early Hunters and Huntresses are trained.
A cool plot could be that a villain from the future uses time travel to go back and change the past in order to gain ultimate power in the Arrow universe. The heroes then have to figure out a way to reverse the changes and stop the villain. This could involve a lot of action scenes as they race against time to save their world from this altered timeline.
One idea could be that the characters from HTTYD find a strange portal that leads them to the Pokemon world. There, they have to learn how to catch and train Pokemon to find their way back home. Hiccup, being the curious and inventive type, might try to combine the technology of Berk with Pokemon training methods.
One idea could be that a human character, perhaps a skilled dragon rider, discovers a hidden group of anthro female dragons. Each dragon has a unique personality and power. They gradually form a bond with the human and together they face threats from other dragon clans. This creates a sense of unity and adventure within the harem dynamic.
One idea could be Boruto traveling back to the era of the First Hokage. He could witness the founding of Konoha and interact with the original founders. Maybe he tries to prevent a certain event that he knows will have long - term negative consequences in his own time.
One interesting plot idea could be Percy traveling back in time to the era of the Greek gods' creation. He could witness the birth of the first gods and goddesses and perhaps even play a role in some of the early events. Another idea is him going back to the time of a great hero like Hercules and joining forces with him to complete a quest that was left unfinished. Or he could travel to the future to see what has become of the demigod world and then come back to the present to try and prevent a bad future from happening.