In most DBZ fanfiction, time travel often works through some sort of magical or technological device. For example, in some stories, Bulma creates a time machine similar to the one in the movie 'Back to the Future'. This allows characters to travel to different points in time within the Dragon Ball Z timeline.
One great 'Time Travel DBZ Fanfiction' is 'Back to the Future Saiyan Style'. It has an interesting plot where Goku travels back in time to prevent a major villain from ever rising to power. The character development is really well - done, and it stays true to the spirit of the Dragon Ball Z universe.
Vegeta is also quite popular. His pride and his constant strive for power can be really interesting when combined with time travel. He might go back in time to prove himself even stronger than he was originally or to correct past mistakes that made him look weak.
One interesting concept is how time travel affects character development. For example, a character might go back in time and change a key event, which then alters their future self in unexpected ways. Another concept could be the exploration of alternate timelines. Maybe a character's time travel creates a whole new reality where different battles play out and relationships shift.
One interesting plotline could be Gohan traveling back in time to prevent certain tragedies. For example, he might go back to stop the death of his father, Goku. This would change the entire course of the Dragon Ball Z story. Another could be Gohan using his knowledge from the future to train his younger self more effectively, leading to a much stronger Gohan at an earlier stage.
Goku is often a common character. Since Gohan has a strong relationship with his father, in time - travel scenarios, they might interact a lot. For example, Gohan could go back in time to save Goku from an early death or to train with him. Vegeta is also a likely character. He might be skeptical of Gohan's time - traveling at first but then get involved in Gohan's plans. Another common character would be Piccolo. He has been Gohan's mentor, and in time - travel stories, he could offer his wisdom and help to Gohan again.
One possible plot could be Future Trunks traveling back in time to prevent a new threat that he discovers in his own time. For example, a powerful and unknown enemy is about to emerge in his future, and he goes back to enlist the help of Goku and the others before it's too late. Another plot could involve Trunks accidentally changing something in the past that has unforeseen consequences in his future, like messing with the timeline of a certain character's power - up, which then leads to a whole new set of challenges when he returns to his own time.
He becomes more resourceful. In his own time, he's used to a certain set of circumstances. But during time travel, he has to adapt quickly to new situations and find ways to use the resources available in the past. For example, he might have to figure out how to use different senzu beans or power - up techniques that are new to him.
Some Flash time travel fanfics play with the idea of time travel via dreams. Barry Allen could enter a dream - like state where he experiences different timelines. It's an interesting take as it blurs the line between reality and the subconscious.