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.