Well, one possibility is that Lee could use his knowledge of the future to prevent some of the tragedies in The Walking Dead world. For example, he might be able to stop certain characters from dying prematurely. He could also try to find a cure for the zombie virus much earlier if he travels back far enough. Maybe he'd team up with different groups in different time periods and change the power dynamics within the story.
Lee's character could become more complex. He might start to question his own actions and decisions in the past if he travels to different points in time. His moral compass could be further tested as he sees the consequences of different choices. For example, if he witnesses the result of a decision he made that led to many deaths, he may try to rectify it in another timeline.
One way could be through some sort of experimental device left behind by the military. Maybe a scientist in the group discovers it and accidentally activates it, sending one or more characters back in time. They could then try to change things that led to the zombie apocalypse in the first place.
Rick would probably be one of the first to interact with Lee. They are both leaders in their own right, so there could be some power dynamics at play. Clementine would also be a key character to interact with him, given their past relationship.
It could also be due to a scientific experiment gone wrong. A group of survivors might be trying to find a cure in a makeshift lab and accidentally create a time - traveling device. This device could send them to various points in time within the story's timeline. They might use this to go back and save important characters who died early on, which would completely change the course of events in the fanfiction.
Survival is also a major theme. Even with time travel, characters still have to figure out how to survive in a world overrun by walkers. They use their knowledge from the future to gain an edge, like knowing where to find safe havens or weapons. Another theme is changing the future. By going back in time, they hope to alter the course of events for the better, whether it's preventing a certain group from turning bad or stopping a major disaster from happening.
The plot of 'The Walking Dead Lee Returns Fanfiction' might involve Lee's return being part of a bigger mystery. He could be followed by a strange group that no one knows about. His memories might be hazy, and he has to piece together what happened to him while he was gone. This could lead to new adventures and challenges for the main characters as they try to figure out Lee's situation and the threat or opportunity that his return might bring.