Gray could be an interesting focus. He might travel back in time and have to hide his ice - make magic as it could be seen as a threat in a different era. He could also meet his family or mentors from the past, which would give him a chance to learn more about his own magic heritage and grow as a mage in a different way compared to his normal development in the present - day Fairy Tail.
Lucy would also be a great choice. Since she has celestial spirit magic, she could potentially use it in unique ways in the past. Maybe she discovers new spirits that are long lost in her time. Also, her intelligence and determination could play a big role in a time - travel story, like trying to figure out how to get back to her own time while also helping the people in the past.
Natsu could be really interesting. Imagine him traveling back in time and accidentally causing chaos because of his impulsive nature. He might try to use his fire magic in a time when it was less understood or accepted, and have to deal with the consequences. He could also meet younger versions of his friends and have heart - warming or hilarious interactions.
Natsu from Fairy Tail would be a great character to focus on. His dragon - slayer magic and hot - headed personality would make for some exciting interactions with the characters from Black Clover. He could have friendly rivalries with the Magic Knights, especially those with powerful fire - based magic. Also, his loyalty to his guild and friends could be a driving force in a story where he has to cooperate with Black Clover characters to save both worlds.
I think Lucy would be really interesting. She could use her Celestial Spirit Magic in creative ways to deal with the situation in the highschool during the apocalypse. Maybe she could call upon spirits to help barricade areas or to scout for danger.
One interesting plot could be Lucy traveling back in time to the founding of Fairy Tail. She could then meet the original members and influence the guild's early development in unexpected ways, like helping to create some of the guild's most important rules.
Well, one interesting idea could be how Nashi's actions in the past through time travel might change the future of Fairy Tail. For example, she could prevent a major disaster that was supposed to happen in the original timeline.
One idea could be a character traveling back in time to prevent a major disaster in the Fairy Tail world, like the destruction of a key guild building. This would involve them having to figure out the cause of the future event in the past and deal with the different versions of their friends and enemies at that time.
I really like the fanfiction titled 'Time - Twisted Tales in Fairy Tail'. In this story, the time - travel aspect is used to explore different relationships between the characters. It dives deep into how changing the past could impact the future of Fairy Tail, with some really cool plot twists.
Natsu is always an interesting character to focus on. His determination and his unique fire - based magic can lead to some exciting adventures on Tenrou Island. He could be on a quest to find a powerful fire - related relic hidden on the island. Another great character is Lucy. Her celestial spirit magic can add a different dimension to the story. She might use her spirits to help her and her friends navigate the dangerous parts of Tenrou Island or solve mysteries there.
Mard Geer is a very interesting character to focus on. His complex relationship with Zeref and his extreme loyalty to Tartaros provide a lot of material for exploration. You could also write about Gray Fullbuster. His struggle with his dark side and his connection to the demons of Tartaros make him a great focal point. Another option is Erza Scarlet. Her strength and determination in the face of the Tartaros threat can lead to some exciting storylines.
Reaper could also be an interesting focus. His dark and mysterious past could be explored further through time travel. He could go back to the time when he was still Gabriel Reyes, before the transformation. This could show the events that led to his current state, and perhaps in a time travel scenario, he could even have a chance to change his fate. It would be a story full of drama, mystery and action as he grapples with his past self and the decisions that led him to become Reaper.
I think Kai would be a great focus. His hot - headed nature could lead to some really chaotic time - travel situations. For instance, if he travels back to a time when the Fire Temple was in danger, he might rush into action without fully understanding the consequences, and end up changing the future of the Fire Element in Ninjago in unexpected ways.